Remove the in memory buffer Lucene store/directory
closes #4994
fix docs to reflect removal of byte buffer memory
Add [0.90.11] version as a constant
Improve Aggregations documentation

* Mostly minor things like typos and grammar stuff
* Some clarifications
* The note on the deprecation was ambiguous. I've removed the problematic part so that it now definitely says it's deprecated
Fix confusing sentence

The original sentence didn't make much sense. I hope this is a bit better. Taken heavy inspiration from https://github.com/elasticsearch/elasticsearch/commit/c63d8c4fb566882bd4655ddc7dec13b4e8719244
Rename `geohashgrid` to `geohash_grid` in documentation.

It was renamed in fc6bc4c4776a2f710f57616e3495aaf6a230c4d3.

Close #4997
Does not support "script" in value_clunt aggregation.
[DOC] Tidied up geohashgrid aggregations
[TEST] thrown error when there are no REST tests to run
[TEST] run REST tests against multiple nodes (round-robin)

Multiple nodes are now started when running REST tests against the `TestCluster` (default randomized settings are now used instead of the hardcoded `1`)

Added also randomized round-robin based on all available nodes, and ability to provide multiple addresses when running tests against an external cluster to have the same behaviour
[TEST] removed leftover delete alias test in delete mapping
[TEST] slightly sped up warmer tests by providing warmers during index creation
[TEST] disabled refresh for get_source realtime test