This commit is the first in a series which will culminate with
fully-functional shard history retention leases.

Shard history retention leases are aimed at preventing shard history
consumers from having to fallback to expensive file copy operations if
shard history is not available from a certain point. These consumers
following indices in cross-cluster replication, and local shard
recoveries. A future consumer will be the changes API.

Further, index lifecycle management requires coordinating with some of
these consumers otherwise it could remove the source before all
consumers have finished reading all operations. The notion of shard
history retention leases that we are introducing here will also be used
to address this problem.

Shard history retention leases are a property of the replication group
managed under the authority of the primary. A shard history
retention lease is a combination of an identifier, a retaining sequence
number, a timestamp indicating when the lease was acquired or required,
and a string indicating the source of the lease. Being leases they have
a limited lifespan that will expire if not renewed. The idea of these
leases is that all operations above the minimum of all retaining
sequence numbers will be retained during merges (which would otherwise
clear away operations that are soft deleted). These leases will be
periodically persisted to Lucene and restored during recovery, and
broadcast to replicas under certain circumstances.

This commit is merely putting the basics in place. This first commit
only introduces the concept and integrates their use with the soft
delete retention policy. We add some tests to demonstrate the basic
management is correct, and that the soft delete policy is correctly
influenced by the existence of any retention leases. We make no effort
in this commit to implement any of the following:
 - timestamps
 - expiration
 - persistence to and recovery from Lucene
 - handoff during primary relocation
 - sharing retention leases with replicas
 - exposing leases in shard-level statistics
 - integration with cross-cluster replication

These will occur individually in follow-up commits.

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