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>