Use collectExistingBucket in GlobalOrdinalsSignificantTermsAggregator.WithHash.

Relates to #5955.
Added `reverse_nested` aggregation.

The `reverse_nested` aggregation allows to aggregate on properties outside of the nested scope of a `nested` aggregation.

Closes #5507
Fixed license headers.
Use non analyzed token stream optimization everywhere
In the string type, we have an optimization to reuse the StringTokenStream on a thread local when a non analyzed field is used (instead of creating it each time). We should use this across the board on all places where we create a field with a String.
Also, move to a specific XStringField, that we can reuse StringTokenStream instead of copying it.
closes #6001
[TEST] Replace execute().actionGet() with get()
[TEST] (RecoveryPercolatorTests) Don't stop the master node and always use the client of the master node
debug log of receiving a cluster state from another master could be erroneously logged

Added trace logging to MinimumMasterNodesTests.multipleNodesShutdownNonMasterNodes
Support empty properties array in mappings
closes #5887
function_score parser throws exception if both functions:[] and single function given

In addition, add a special warning if the misplaced function is a "boost_factor"
function to avoid confusion of "boost" and "boost_function".

closes #5995
Update JNA to latest version

Updating to this version allows to configure a special JNA directory,
in case the /tmp directory is mounted with the noexec option, as JNA
extracts some data and tries to execute parts of it.

Also updated documentation to clarify mlockall and memory settings as well
as pointing to the new jna.tmpdir system property.

Closes #5493
[DOCS] Added Twitter Storehaus client

Added Twitter Storehaus client
Fix behavior on default boost factor for More Like This.

A boost terms factor of 1.0 is not the same as no boosting of terms.
The desired behavior is to deactivate boosting by default. If the user
specifies any value other than 0, then boosting is activated.

Closes #6021
Do not start a recovery process if the primary shard is currently allocated on a node which is not part of the cluster state

If a source node disconnect during recover, the target node will respond by canceling the recovery. Typically the master will respond by removing the disconnected node from the cluster state, promoting another shard to become primary. This is sent it to all nodes and the target node will start recovering from the new primary. However, if the drop of a node caused the node count to go bellow min_master_node, the master will step down and will not promote shard immediately. When a new master is elected we may publish a new cluster state (who's point is to notify of a new master) which is not yet updated. This caused the node to start a recovery to a non existent node. Before we aborted the recovery without cleaning up the shard, causing subsequent correct cluster states to be ignored. We should not start the recovery process but wait for another cluster state to come in.

Closes #6024
[DOC] Fix typo in function score query documentation.
[TEST] Replace folded blocks with literal blocks

The regex tests are formatted with blocks for readability.  Previously,
they were formatted using folded style blocks (e.g. using `>`). Folded
blocks convert newlines into spaces.  This is problematic for our regex,
since comments can only be terminated with a newline.

Effectively, anything after a comment will be commented out, making many
of the regex "silently pass".

This commit replaces them with scalar-style blocks (e.g. using `|`), which
treats newlines as significant, and thus correctly terminates comments
inside the regex.

Also fixes a regex test (`cat.thread_pool/10_basic.yaml`) that started
to fail after the block was fixed.  The test was missing a `\s+` before
the closing newline.
[TEST] Disable REST tests until #6033 is fixed.
Change default filter cache to 10% and circuit breaker to 60%
The defaults we have today in our data intensive memory structures don't properly add up to properly protected from potential OOM.
The circuit breaker, today at 80%, aims at protecting from extensive field data loading. The default threshold today is too permissive and can still cause OOMs.
 The filter cache today is at 20%, and its too high when adding it to other limits we have, reduce it to 10%, which is still a big enough portion of the heap, yet provides improved safety measure.
 closes #5990
Fix StackOverflowException for long suggestion strings

Changed getFiniteStrings to use an iterative implementation instead of
recursive, so we don't use a Java stack-frame per character for each
suggestion at build & query time.
[TEST] fixed _cat/thread_pool REST tests with local transport, in case the transport port is not available and gets returned as '-'

Re-enabled REST tests suite

Closes #6033
Search might not return on thread pool rejection
When a thread pool rejects the execution on the local node, the search might not return.
This happens due to the fact that we move to the next shard only *within* the execution on the thread pool in the start method. If it fails to submit the task to the thread pool, it will go through the fail shard logic, but without "counting" the current shard itself. When this happens, the relevant shard will then execute more times than intended, causing the total opes counter to skew, and for example, if on another shard the search is successful, the total ops will be incremented *beyond* the expectedTotalOps, causing the check on == as the exit condition to never happen.
The fix here makes sure that the shard iterator properly progresses even in the case of rejections, and also includes improvement to when cleaning a context is sent in case of failures (which were exposed by the test).
Though the change fixes the problem, we should work on simplifying the code path considerably, the first suggestion as a followup is to remove the support for operation threading (also in broadcast), and move the local optimization execution to SearchService, this will simplify the code in different search action considerably, and will allow to remove the problematic #firstOrNull method on the shard iterator.
The second suggestion is to move the optimization of local execution to the TransportService, so all actions will not have to explicitly do the mentioned optimization.
fixes #4887