[Docs] added a missing reference to significantterms-aggergations

Also fix header level mismatch issue reported by the build
[DOCS] Updating scripting docs for geo functions

Added a few functions are corrected the default unit where necessary
[DOCS] Fix typo in the reference doc. SuSe -> SUSE

SUSE, as a Linux distribution, is never lower cased

fixes #5354
[DOCS] Reword clear-scroll sentence
[DOCS] Fixing contributing.md indentation

Add a whitespace to make these separate paragraphs inside of a list.
Documentation fix for significant_terms heading levels
[TEST] Use client node such that we always have to do a round-trip to do a fetch.
Move systemd files from /etc to /usr/lib

As documented in systemd's manual pages tmpfiles.d(5) and systemd.unit(5),
a package should install its default configuration in /usr/lib, which can
be overriden by system administrators in /etc.

New locations in the rpm:
/usr/lib/systemd/system/elasticsearch.service
/usr/lib/tmpfiles.d/elasticsearch.conf
Remove plugin isolation feature for a future version

relates #5261
cleanup formatting in significant aggs package
[TEST] Print tests.jvms and tests.client.ratio if set

We need to print options that can modify stream of events we need
to print it otherwise a seed might not reproduce the failure in
the tests.
[DOCS] Add Azure storage repositories
(cherry picked from commit 0805c01)
[TEST] pre check download service working

Seen during CI tests, it could appears that the download service is not available for any reason.

This fix in test will check before each test which requires an internet access (annotated with @Network) that the download service we are testing is still working.

It won't fail the test but will mark the test as `Ignored` in case of failure.
(cherry picked from commit 8dfdc6f)
Findbug warning supression

Added logic to enable findbug warnings supression via annotations
Revert "Findbug warning supression"

This reverts commit 8e9f8c2409a0c777316ad6c26b815d3c49399923.
Add ability to get snapshot status for running snapshots

Closes #4946
Made p/c override the clone() method. This is necessary since by default clone will make a shallow copy of the original object, while for p/c queries we need to make sure that the wrapped queries are also cloned.
Make indexRandom handle many documents better

* Index one at a time only rarely if doing more then 300.
* When launching async actions, take some care to make sure you don't already
have more then 150 other async actions in flight.
* When indexing in bulk split into chunks of 1000 documents.
Speed up phrase suggestion scoring

Two changes:
1.  In the StupidBackoffScorer only look for the trigram if there is a bigram.
2.  Cache the frequencies in WordScorer so we don't look them up again and
again and again.  This is implemented by wrapping the TermsEnum in a special
purpose wrapper that really only works in context of the WordScorer.

This provides a pretty substantial speedup when there are many candidates.

Closes #5395
Move to use serial merge schedule by default
Today, we use ConcurrentMergeScheduler, and this can be painful since it is concurrent on a shard level, with a max of 3 threads doing concurrent merges. If there are several shards being indexed, then there will be a minor explosion of threads trying to do merges, all being throttled by our merge throttling.
Moving to serial merge scheduler will still maintain concurrency of merges across shards, as we have the merge thread pool that schedules those merges. It will just be a serial one on a specific shard.
Also, on serial merge scheduler, we now have a limit of how many merges it will do at one go, so it will let other shards get their fair chance of merging. We use the pending merges on IW to check if merges are needed or not for it.
Note, that if a merge is happening, it will not block due to a sync on the maybeMerge call at indexing (flush) time, since we wrap our merge scheduler with the EnabledMergeScheduler, where maybeMerge is not activated during indexing, only with explicit calls to IW#maybeMerge (see Merges).
closes #5447