Introduces support for multiple host providers, which allows the settings based hosts resolver to be
treated just as any other UnicastHostsProvider. Also introduces the notion of a HostsResolver so
that plugins such as FileBasedDiscovery do not need to create their own thread pool for resolving
hosts, making it easier to add new similar kind of plugins.

* master:
  Add get field mappings to High Level REST API Client (#31423)
  [DOCS] Updates Watcher examples for code testing (#31152)
  TEST: Add bwc recovery tests with synced-flush index
  [DOCS] Move sql to docs (#31474)
  [DOCS] Move monitoring to docs folder (#31477)
  Core: Combine doExecute methods in TransportAction (#31517)
  IndexShard should not return null stats (#31528)
  fix repository update with the same settings but different type (#31458)
  Fix Mockito trying to mock IOException that isn't thrown by method (#31433) (#31527)
  Node selector per client rather than per request (#31471)
  Core: Combine messageRecieved methods in TransportRequestHandler (#31519)
  Upgrade to Lucene 7.4.0. (#31529)
  [ML] Add ML filter update API (#31437)
  Allow multiple unicast host providers (#31509)
  Avoid deprecation warning when running the ML datafeed extractor. (#31463)
  REST high-level client: add simulate pipeline API (#31158)
  Get Mapping API to honour allow_no_indices and ignore_unavailable (#31507)
  [PkiRealm] Invalidate cache on role mappings change (#31510)
  [Security] Check auth scheme case insensitively (#31490)
  In NumberFieldType equals and hashCode, make sure that NumberType is taken into account. (#31514)
  [DOCS] Fix REST tests in SQL docs
  [DOCS] Add code snippet testing in more ML APIs (#31339)
  Core: Remove ThreadPool from base TransportAction (#31492)
  [DOCS] Remove fixed file from build.gradle
  Rename createNewTranslog to fileBasedRecovery (#31508)
  Test: Skip assertion on windows
  [DOCS] Creates field and document level security overview (#30937)
  [DOCS] Significantly improve SQL docs
  [DOCS] Move migration APIs to docs (#31473)
  Core: Convert TransportAction.execute uses to client calls (#31487)
  Return transport addresses from UnicastHostsProvider (#31426)
  Ensure local addresses aren't null (#31440)
  Remove unused generic type for client execute method (#31444)
  Introduce http and tcp server channels (#31446)

* 6.x:
  Avoid sending duplicate remote failed shard requests (#31313)
  Add get field mappings to High Level REST API Client Relates to #27205
  [DOCS] Updates Watcher examples for code testing (#31152)
  [DOCS] Move monitoring to docs folder (#31477)
  [DOCS] Fixes SQL docs in nav
  [DOCS] Move sql to docs
  IndexShard should not return null stats - empty stats or AlreadyCloseException if it's closed is better
  Clarify that IP range data can be specified in CIDR notation. (#31374)
  Remove some cases in FieldTypeLookupTests that are no longer relevant. (#31381)
  In NumberFieldType equals and hashCode, make sure that NumberType is taken into account. (#31514)
  fix repository update with the same settings but different type
  Revert "AwaitsFix FullClusterRestartIT#testRecovery"
  Upgrade to Lucene 7.4.0. (#31529)
  Avoid deprecation warning when running the ML datafeed extractor. (#31463)
  Retry synced-flush in FullClusterRestartIT#testRecovery
  Allow multiple unicast host providers (#31509)
  [ML] Add ML filter update API (#31437)
  AwaitsFix FullClusterRestartIT#testRecovery
  Fix missing historyUUID in peer recovery when rolling upgrade 5.x to 6.3 (#31506)
  Remove QueryCachingPolicy#ALWAYS_CACHE (#31451)
  Rename createNewTranslog to fileBasedRecovery (#31508)
  [DOCS] Add code snippet testing in more ML APIs (#31339)
  [DOCS] Remove fixed file from build.gradle
  [DOCS] Creates field and document level security overview (#30937)
  Test: Skip assertion on windows
  [DOCS] Move migration APIs to docs (#31473)
  Add a known issue for upgrading from 5.x to 6.3.0 (#31501)
  Return transport addresses from UnicastHostsProvider (#31426)
  Add Delete Snapshot High Level REST API
  Reload secure settings for plugins (#31481)
  [DOCS] Fix JDBC Maven client group/artifact ID

* elastic/master: (92 commits)
  Reduce number of raw types warnings (elastic#31523)
  Migrate scripted metric aggregation scripts to ScriptContext design (elastic#30111)
  turn GetFieldMappingsResponse to ToXContentObject (elastic#31544)
  Close xcontent parsers (partial) (elastic#31513)
  Ingest Attachment: Upgrade Tika to 1.18 (elastic#31252)
  TEST: Correct the assertion arguments order (elastic#31540)
  Add get field mappings to High Level REST API Client (elastic#31423)
  [DOCS] Updates Watcher examples for code testing (elastic#31152)
  TEST: Add bwc recovery tests with synced-flush index
  [DOCS] Move sql to docs (elastic#31474)
  [DOCS] Move monitoring to docs folder (elastic#31477)
  Core: Combine doExecute methods in TransportAction (elastic#31517)
  IndexShard should not return null stats (elastic#31528)
  fix repository update with the same settings but different type (elastic#31458)
  Fix Mockito trying to mock IOException that isn't thrown by method (elastic#31433) (elastic#31527)
  Node selector per client rather than per request (elastic#31471)
  Core: Combine messageRecieved methods in TransportRequestHandler (elastic#31519)
  Upgrade to Lucene 7.4.0. (elastic#31529)
  [ML] Add ML filter update API (elastic#31437)
  Allow multiple unicast host providers (elastic#31509)
  ...