Add docs for plugin isolation

(cherry picked from commit 9624b215fb6e8e9d646a9375b68f64ec8549cff0)
[TEST] Fix SearchStatsTests to have all shards allocated

If randomization brings up a single shard per index in this test
we might run our searches on only one index which causes the assertions
to fail afterwards that's why we need to wait until everything is alloated.
[TEST] Beef up SearchStatsTests
[TEST] Make sure that a snapshot is completed before trying to modify repository
[TEST] fixed SimpleQueryTests#testMultiMatchQuery check for shard failures

It can happen that not all shards are ready, thus we won't have a total failure, but we do need to check that we have at least a failure. Checked also the message of the failure.
Use patched version of TermsFilter to prevent using wrong cached results

See LUCENE-5502

Closes #5363
Added more logging
Keep track of the exceptions instead of just flagging that an exception has occured.
Blocking writes on a tribe node creates a "blocks" tribe
fixes #5389
Corrected issue with throttle type setting not respected upon updates
[TEST] stabelize SearchStatsTests
[Test] Fix missing release.
[Build] Remove XReferenceManager and XSearcherManager from forbidden-apis' exclude list.

These classes have been removed on the upgrade to Lucene 4.7.
Disable by default plugins isolation
compare with .bytes() instead of ByteSizeValue.equals() in breaker service
[TESTS] Fix incorrect discovery options.
[DOCS] Added spring data elasticsearch integration
[DOCS] Various aggregation doc fixes
For unicast zen discovery don't overwrite a ping response for a node if the previous ping response has a set master and the current response hasn't.

Per single main ping request we maintain the received ping response per node. Each node level ping response is mapped into that. If from a previous node level ping request the response has already been set for a node, it will be overwritten. We give higher value to the latest response. This change makes sure that this doesn't happen if the previous response has a set master and the current response hasn't a set master. Otherwise a node will lose the fact that another node has elected itself as master, the result of that would be that there would multiple master nodes in a single cluster.

Closes #5413
[TESTS] Added AwaitsFix