* elastic/master: (43 commits)
  Remove remaining occurances of "include_type_name=true" in docs (elastic#37646)
  SQL: Return Intervals in SQL format for CLI (elastic#37602)
  Publish to masters first (elastic#37673)
  Un-assign persistent tasks as nodes exit the cluster (elastic#37656)
  Fail start of non-data node if node has data (elastic#37347)
  Use cancel instead of timeout for aborting publications (elastic#37670)
  Follow stats api should return a 404 when requesting stats for a non existing index (elastic#37220)
  Remove deprecated FieldNamesFieldMapper.Builder#index (elastic#37305)
  Document that date math is locale independent
  Bootstrap a Zen2 cluster once quorum is discovered (elastic#37463)
  Upgrade to lucene-8.0.0-snapshot-83f9835. (elastic#37668)
  Mute failing test
  Fix java time formatters that round up (elastic#37604)
  Removes awaits fix as the fix is in. (elastic#37676)
  Mute failing test
  Ensure that max seq # is equal to the global checkpoint when creating ReadOnlyEngines (elastic#37426)
  Mute failing discovery disruption tests
  Add note about how the body is referenced (elastic#33935)
  Define constants for REST requests endpoints in tests (elastic#37610)
  Make prepare engine step of recovery source non-blocking (elastic#37573)
  ...

This commit changes the TransportVerifyShardBeforeCloseAction so that it issues a 
forced flush, forcing the translog and the Lucene commit to contain the same max seq 
number and global checkpoint in the case the Translog contains operations that were 
not written in the IndexWriter (like a Delete that touches a non existing doc). This way 
the assertion added in #37426 won't trip.

Related to #33888

This commit changes the TransportVerifyShardBeforeCloseAction so that it issues a 
forced flush, forcing the translog and the Lucene commit to contain the same max seq 
number and global checkpoint in the case the Translog contains operations that were 
not written in the IndexWriter (like a Delete that touches a non existing doc). This way 
the assertion added in #37426 won't trip.

Related to #33888

… ReadOnlyEngines (elastic#37426)

Since version 6.7.0 the Close Index API guarantees that all translog 
operations have been correctly flushed before the index is closed. If 
the index is reopened as a Frozen index (which uses a ReadOnlyEngine) 
we can verify that the maximum sequence number from the last Lucene 
commit is indeed equal to the last known global checkpoint and refuses 
to open the read only engine if it's not the case. In this PR the check is 
only done for indices created on or after 6.7.0 as they are guaranteed 
to be closed using the new Close Index API.

Related elastic#33888

…8727)

The Close Index API has been refactored in 6.7.0 and it now performs
pre-closing sanity checks on shards before an index is closed: the maximum
sequence number must be equals to the global checkpoint. While this is a
strong requirement for regular shards, we identified the need to relax this
check in the case of CCR following shards.

The following shards are not in charge of managing the max sequence
number or global checkpoint, which are pulled from a leader shard. They
also fetch and process batches of operations from the leader in an unordered
way, potentially leaving gaps in the history of ops. If the following shard lags
a lot it's possible that the global checkpoint and max seq number never get
in sync, preventing the following shard to be closed and a new PUT Follow
action to be issued on this shard (which is our recommended way to
resume/restart a CCR following).

This commit allows each Engine implementation to define the specific
verification it must perform before closing the index. In order to allow
following/frozen/closed shards to be closed whatever the max seq number
or global checkpoint are, the FollowingEngine and ReadOnlyEngine do
not perform any check before the index is closed.

Co-authored-by: Martijn van Groningen <martijn.v.groningen@gmail.com>

This commit also contains #37426.
Related #33888