We've recently seen a number of test failures that tripped an assertion in IndexShard (see issues
linked below), leading to the discovery of a race between resetting a replica when it learns about a
higher term and when the same replica is promoted to primary. This commit fixes the race by
distinguishing between a cluster state primary term (called pendingPrimaryTerm) and a shard-level
operation term. The former is set during the cluster state update or when a replica learns about a
new primary. The latter is only incremented under the operation block, which can happen in a
delayed fashion. It also solves the issue where a replica that's still adjusting to the new term
receives a cluster state update that promotes it to primary, which can happen in the situation of
multiple nodes being shut down in short succession. In that case, the cluster state update thread
would call `asyncBlockOperations` in `updateShardState`, which in turn would throw an exception
as blocking permits is not allowed while an ongoing block is in place, subsequently failing the shard.
This commit therefore extends the IndexShardOperationPermits to allow it to queue multiple blocks
(which will all take precedence over operations acquiring permits). Finally, it also moves the primary
activation of the replication tracker under the operation block, so that the actual transition to
primary only happens under the operation block.
Relates to #32431, #32304 and #32118
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