[Infra] re-send failed shard messages when receiving a cluster state that still refers to them

In rare cases we may fail to send a shard failure event to the master, or there is no known master when the shard has failed (ex. a couple of node leave the cluster canceling recoveries and causing a master to step down at the same time). When that happens and a cluster state arrives from the (new) master we should resend the shard failure in order for the master to remove the shard from this node.

Closes #6881
[Recovery] don't start a gateway recovery if source node is not found

Due to change introduced in #6825, we now start a local gateway recovery for replicas, if the source node can not be found. The recovery then fails because we never recover replicas from disk.

Closes #6879
[TESTS] Stabilize DisabledFieldDataFormatTests by setting number_of_replicas to 0.
[DOCS] : Indexed scripts/templates

These are the docs for the indexed scripts/templates feature.
Also moved the namespace for the REST endpoints.

Closes #6851
[DOCS][FIX] Fix doc parsing, broken closing block
[DOCS][FIX] Fix reference check in indexed scripts/templates doc.
Aggregations: Fixed Histogram key_as_string bug

The key as string field in the response for the histogram aggregation will now only show if format is specified on the request.

Closes #6655
[TEST] Verify if clear cache request went to all shards.
[DOCS] Move benchmark API to 1.4
Threadpool Info: Allow to serialize negative thread pool sizes

As a SizeValue is used for serializing the thread pool size, a negative number
resulted in throwing an exception when deserializing (using -ea an assertionerror
was thrown).

This fixes a check for changing the serialization logic, so that negative numbers are read correctly, by adding an internal UNBOUNDED value.

Closes #6325
Closes #5357