Introduced a new IMMEDIATE priority - higher than URGENT

* applied to cluster update settings, reroute, node join/leave events, node failure

Closes #5062
[TEST] Replaced RestApiParser assertions with exceptions for better error messages
[DOC] The percentiles aggregation is coming in 1.1.0.
Fix small typo in percentiles doc
[DOCS] Multiple doc fixes

Closes #5047
Display all available endpoints by default at /_cat

Closes #5106
Increase RamAccountingTermsEnum flush size from 1mb to 5mb

Reduces the number of logs when TRACE logging is turned on for the
circuit breaker
[DOCS] Updated the Perl docs
mvel: upgrade to 2.1.9
jackson: upgrade to 2.3.2
Don't throw UOE in PercolateContext#from and #size
Create mapping in PercolatorTests#testPercolateSorting_unsupportedField in create index call instead of lazily via index call.
Also make use of the thread local memory reuse for a document being percolated with nested objects.
The memory index will only be reused for the root doc, since most of the times that will be the biggest document.
[DOCS] Add `coming` tag to multiple rescores

Closes #5365
Improve error detection in geo_filter parsing

Relates to #5370
[TEST] Randomized number of shards used for indices created during tests

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
- adapted tests that didn't need a specific number of shards to the new random behaviour
- fixed a couple of test bugs (e.g. 3 levels parent child test could only work on a single shard as the routing key used for grand-children wasn't correct)
- also done some cleanup, shared code through shard size facets and aggs tests and used common methods like `assertAcked`, `ensureGreen`, `refresh`, `flush` and `refreshAndFlush` where possible
- made sure that `indexSettings()` is always used as a basis when using `prepareCreate` to inject specific settings
- converted indexRandom(false, ...) + refresh to indexRandom(true, ...)
[TEST] Moving mapping creation to create index call, this will make sure that in the test the mapping is always available on all nodes.