Scripts: Return new lists on calls to getValues.

Scripts currently share the same list across invocations to getValues. This
caused a bug in script fields where all documents coming from the same segment
would get the same values (basically, for the next document for which script
values have been requested). Scripts now return a fresh new list on every
invocation to `getValues`.

Close #8576
Added note that ES packages automatically change vm.max_map_count

Closes #8601
Spec: Fixes URL links to documentation

Closes #8656
Docs: Adds documentation for indices.exists_template

Closes: #8657
Update to Jackson 2.4.3

- Update pom to 2.4.3 from 2.4.2
- Enable the CBOR data header (aka tag) from the CBOR Generator to provide binary identification like the Smile format
- Check for the CBOR header and ensure that the data sent in represents a "major type" that is an object
- Cleans up `JsonVsCborTests` unused imports
Fix for geohash neighbors when geohash length is even.
We don't have to set XLimit and YLimit depending on the level (even or odd), since semantics of x and y are already swapped on each level.
XLimit is always 7 and YLimit is always 3.

Close #8526
Adding unit test for even / odd boundary condition
Revert "Update to Jackson 2.4.3"

This reverts commit 7523d0b150ff7b05aa37b5e6edf8216341671b38.
Core: clarify index removal log message
Raise REST test Timeout - LocalGW takes it't tall...
Core: Let the disk threshold decider take into account shards moving away from a node in order to determine if a shard can remain.

By taking this into account we can prevent that we move too many shards away than is necessary.

Closes #8538
Closes #8659
Core: separately log file deletions

Today, you can turn on lucene.iw TRACE logging, but that produces tons
of output.  This changes breaks out separate lucene.iw.ifd and
index.store.deletes logger components (TRACE), disabled by default, to
see what part of Elasticsearch is deleting index files.

Closed #8662

Closed #8603
scripts: add explainable script again

explainable scripts were removed in #7245 but they were used.
This commit adds them again.

closes #8561