[TEST] when the test fail, have the exception message as the reason
the test failed but couldn't repro (yet), at the very least, make sure we have the exception message as the reason, can help to track down the failure itself when it happens again
Make sure we don't reuse arrays when sending and error back
We want to make sure recycling will not fail for any reason while trying to send a response back that is caused by a failure, for example, if we have circuit breaker on it (at one point), sending an error back will not be affected by it.
closes #6631
Thread pool rejection status code should be 429
Thread rejection should return too many requests status code, and not 503, which is used to also show that the cluster is not available
 relates to #6627, but only for rejections for now
closes #6629
[STORE] Wrap RateLimiter rather than copy RateLimitedIndexOutput

We clone RateLimitedIndexOutput from lucene just to collect pausing
statistics we can do this in a more straight forward way in a delegating
RateLimiter.

Closes #6625
[TEST] Change es.node.mode default for tests to `local`

In order to speed up test execution we should run in local mode by
default. CI builds will still use network builds all the time.

Closes #6624
Disable explicit GC by default

We don't rely upon GC to cleanup mappedbytebuffers, we unmap them
explicitly on close in lucene. But the JDK has crazy loops with
explicit GCs in exceptional cases to try to force unmapping.

In general we don't want any of our code or library code calling
this method: so its banned in forbidden-apis as well.
Added minimal setup guide for BW Compat tests
[TEST] Testcase for #6639
[Test] testRelocationWhileRefreshing should wait for the first shard to be started
Recovery API should also report ongoing relocation recoveries

We currently only report relocation related recoveries after they are done.

Closes #6585
[Test] add awaitFix to rerouteRecoveryTest
[Test] improved recovery slow down in rerouteRecoveryTest

only change recovery throttling to slow down recoveries. The recovery file chunk size updates are not picked up by ongoing recoveries. That cause the recovery to take too long even after the default settings are restored.

Also - change document creation to reuse field names in order to speed up the test.
[Test] make recovery slow down in rerouteRecoveryTest aware of index size
[TEST] Randomize netty worker and connection parameters
Try and push our system to a state where there is only a single worker, trying to expose potential deadlocks when we by mistake execute blocking operations on the worker thread
closes #6635
Wrap groovy script exceptions in a serializable Exception object

Fixes #6598
Update mapping on master in async manner
Today, when a new mapping is introduced, the mapping is rebuilt (refreshSource) on the thread that performs the indexing request. This can become heavier and heavier if new mappings keeps on being introduced, we can move this process to another thread that will be responsible to refresh the source and then send the update mapping to the master (note, this doesn't change the semantics of new mapping introduction, since they are async anyhow).
When doing so, the thread can also try and batch as much updates as possible, this is handy especially when multiple shards for the same index exists on the same node. An internal setting that can control the time to wait for batches is also added (defaults to 0).

Testing wise, a new support method on ElasticsearchIntegrationTest#waitForConcreteMappingsOnAll to allow to wait for the concrete manifestation of mappings on all relevant nodes is added. Some tests mistakenly rely on the fact that there are no more pending tasks to mean mappings have been updated, so if we see, timing related, failures down later (all tests pass), then those will need to be fixed to wither awaitBusy on the master for the new mapping, or in the rare case, wait for the concrete mapping on all the nodes using the new method.
closes #6648

allow to change the additional time window dynamically

better sorting on mappers when refreshing source
also, no need to call nodes info in test, we already have the node names

clean calls to mapping update to provide doc mapper and UUID always
also use the internal cluster support method to get the list of nodes an index is on

reverse the order to pick the latest change first

remove unused field

and fix constructor param

move to start/stop on mapping update action

randomize INDICES_MAPPING_ADDITIONAL_MAPPING_CHANGE_TIME
[TEST] properly wait for mappings when needed
[TEST] properly wait for mapping on master node
add helper method to do so, by not assuming that the mapping will exists right away by waiting for green or refreshing...
Recovery from local gateway should re-introduce new mappings
The delayed mapping intro tests exposed a bug where if a new mapping is introduced, yet not updated on the master, and a full restart occurs, reply of the transaction log will not cause the new mapping to be re-introduced.
closes #6659

add comment on the method
Add ability to restore partial snapshots

Closes #5742