Core: Use FixedBitSetFilterCache for delete-by-query.

Leftover from #7037.
Close #7581
Bulk UDP: Removal.

This feature is rarely used. Removing it will help reduce the moving parts
of Elasticsearch and focus on the core.

Close #7595
Translog: remove unused stream

Closes #7683
Docs: Fix misnamed setting

The settings is `index.merge.policy.reclaim_deletes_weight` not
`index.reclaim_deletes_weight`.

Closes #7676
Add inline comment to prevent confusion
Docs: Added recommended Java versions to the docs
Docs: A simple typo.

Closes #7535
Resiliency: Master election should demotes nodes which try to join the cluster for the first time

With the change in #7493,  we introduced a pinging round when a master nodes goes down. That pinging round helps validating the current state of the cluster and takes, by default, 3 seconds. It may be that during that window, a new node tries to join the cluster and starts pinging (this is typical when you quickly restart the current master).  If this node gets elected as the new master it will force recovery from the gateway (it has no in memory cluster state), which in turn will cause a full cluster shard synchronisation. While this is not a problem on it's own, it's a shame. This commit demotes "new" nodes during master election so the will only be elected if really needed.

Closes #7558
Indices API: Added GET Index API

Returns information about settings, aliases, warmers, and mappings. Basically returns the IndexMetadata. This new endpoint replaces the /{index}/_alias|_aliases|_mapping|_mappings|_settings|_warmer|_warmers and /_alias|_aliases|_mapping|_mappings|_settings|_warmer|_warmers endpoints whilst maintaining the same response formats.  The only exception to this is on the /_alias|_aliases|_warmer|_warmers endpoint which will now return a section for 'aliases' or 'warmers' even if no aliases or warmers exist. This backwards compatibility change is documented in the reference docs.

Closes #4069
[DOCS] Additional documentation for _score accessing

Closes #7043
[TEST] added exception for GET index API to bwc tests
Internal: refactor copy headers mechanism to not require a client factory

With #7594 we replaced the static `BaseRestHandler#addUsefulHeaders` by introducing the `RestClientFactory` that can be injected and used to register the relevant headers. To simplify things, we can now register relevant headers through the `RestController` and remove the `RestClientFactory` that was just introduced.

Closes #7675