Always restore the ThreadContext for operations delayed due to a block (

elastic#23349)

The IndexShardOperationsLock has a mechanism to delay operations if there is currently a block on the lock. These
delayed operations are executed when the block is released and are executed by a different thread. When the different
thread executes the operations, the ThreadContext is that of the thread that was blocking operations. In order to
preserve the ThreadContext, we need to store it and wrap the listener when the operation is delayed.

Update java.asciidoc (elastic#23035)

A couple more minor errors

Update Painless Loop Counter to be Higher (elastic#22560)

Updated Painless loop counter to be 1000000 statements instead of 10000 for update queries.

Only update DocumentMapper if field type changes (elastic#22165)

Merging mappings ensures that fields are used consistently across mapping types. Disabling norms for a specific field in one mapping type for example also disables norms for the same field in other mapping types of that index. The logic that ensures this while merging mappings currently always creates a fresh document mapper for all existing mapping types, even if no change occurred. Creating such a fresh document mapper does not come for free though as it involves recompressing the source. Making a mapping change to one type of an index with 100 types will thus re-serialize and recompress all 100 types, independent of any changes made to those types.

This commit fixes the update logic to only create a new DocumentMapper if a field type actually changes.

Fixed bad asciidoc link

Document thread-safety for ingest processors

With this commit we document that ingest processors need to be
thread-safe. Previously this could be inferred from reading the
source code but we got several user questions about this so
it is stated explicitly in the Javadocs of Processor now.

Build: Apply license section in poms only to elasticsearch artifacts (e…

…lastic#21757)

release [1.7.6]

Remove cluster update task when task times out (elastic#21578)

Fixes an issue where the cluster service does not remove an update task from its internal data structures that are used for batching cluster state updates.