[DOCS] Better document the limitation of nested objects.
Percentiles aggregation.

A new metric aggregation that can compute approximate values of arbitrary
percentiles.

Close #5323
Fix test bug: a too low compression level can make accuracy terrible.
[DOCS] Java API JSON typo
Terms aggs: only use ordinals on low-cardinality fields by default.

Close #5303
Fix sorting of NaN values in terms aggregations.

Closes #5236.
Added unit tests for terms and histogram sorting on sub aggregations that return NaN.
Fix NPE/AIOOBE when building a bucket which has not been collected.

Close #5048
Fix `metric` impl on unmapped fields.
Rewrite BytesStreamOutput on top of BigArrays/ByteArray.

Fix for #5159
check for null term in PagedBytesEstimator.bytesPerValue()

Fixes #5326
Implement custom circuit breaker estimator for parent/child field data
Force merges to not happen when indexing a doc / flush
Today, even though our merge policy doesn't return new merge specs on SEGMENT_FLUSH, merge on the scheduler is still called on flush time, and can cause merges to stall indexing during merges. Both for the concurrent merge scheduler (the default) and the serial merge scheduler. This behavior become worse when throttling kicks in (today at 20mb per sec).

In order to solve it (outside of Lucene for now), we wrap the merge scheduler with an EnableMergeScheduler, where, on the thread level, using a thread local, the call to merge can be enabled/disabled.

A Merges helper class is added where all explicit merges operations should go through. If the scheduler is the enabled one, it will enable merges before calling the relevant explicit method call. In order to make sure Merges is the only class that calls the explicit merge calls, the IW variant of them is added to the forbidden APIs list.

closes #5319
[DOCS] add Elasticsearch Image Plugin
Fix issue where circuit breaker was always reset to 80% upon startup
[DOCS] Formatting error

Closes #5346
if there is no master node id, make sure not to fail the cat call, and report x
[DOCS] Update boolean mapping docs with a full list of values that are treated as false

Closes #5337
Added support for sorting buckets based on sub aggregations

 Supports sorting on sub-aggs down the current hierarchy. This is supported as long as the aggregation in the specified order path are of a single-bucket type, where the last aggregation in the path points to either a single-bucket aggregation or a metrics one. If it's a single-bucket aggregation, the sort will be applied on the document count in the bucket (i.e. doc_count), and if it is a metrics type, the sort will be applied on the pointed out metric (in case of a single-metric aggregations, such as avg, the sort will be applied on the single metric value)

 NOTE: this commit adds a constraint on what should be considered a valid aggregation name. Aggregations names must be alpha-numeric and may contain '-' and '_'.

 Closes #5253
[TEST] Randomized number of shards used while testing

Introduced two levels of randomization for the number of shards (between 1 and 10) when running tests:

1) through the existing random index template, which now sets a random number of shards that is shared across all the indices created in the same test method unless overwritten

2) through `createIndex` and `prepareCreate` methods, similar to what happens using the `indexSettings` method, which changes for every `createIndex` or `prepareCreate` unless overwritten (overwrites index template for what concerns the number of shards)

Added the following facilities to deal with the random number of shards:
- `getNumShards` to retrieve the number of shards of a given existing index, useful when doing comparisons based on the number of shards and we can avoid specifying a static number. The method returns an object containing the number of primaries, number of replicas and the total number of shards for the existing index

- added `assertFailures` that checks that a shard failure happened during a search request, either partial failure or total (all shards failed). Checks also the error code and the error message related to the failure. This is needed as without knowing the number of shards upfront, when simulating errors we can run into either partial (search returns partial results and failures) or total failures (search returns an error)

- added common methods similar to `indexSettings`, to be used in combination with `createIndex` and `prepareCreate` method and explicitly control the second level of randomization: `numberOfShards`, `minimumNumberOfShards` and `maximumNumberOfShards`. Added also `numberOfReplicas` despite the number of replicas is not randomized (default not specified but can be overwritten by tests)

Tests that specified the number of shards have been reviewed and the results follow:
- removed number_of_shards in node settings, ignored anyway as it would be overwritten by both mechanisms above
- remove specific number of shards when not needed
- removed manual shards randomization where present, replaced with ordinary one that's now available
- also done some cleanup and used common methods like `assertAcked`, `ensureGreen`, `refresh`, `flush` and `refreshAndFlush` where possible