Be more diligent about ThreadPools having names

Add a name parameter to what was the empty ThreadPool constructor. Assert if the the ThreadPool's setting doesn't contain a name.
DOC:Added field data circuit breaker settings
Remove MVEL as a built-in scripting language
Add a transformer to translate constant BigDecimal to double
Upgrade to Lucene 4.9 (closes #6623)
[TEST] suppress sysout checks since CI runs with debug enabled
[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
[STORE]: Make use of Lucene build-in checksums

Since Lucene version 4.8 each file has a checksum written as it's
footer. We used to calculate the checksums for all files transparently
on the filesystem layer (Directory / Store) which is now not necessary
anymore. This commit makes use of the new checksums in a backwards
compatible way such that files written with the old checksum mechanism
are still compared against the corresponding Alder32 checksum while
newer files are compared against the Lucene build in CRC32 checksum.

Since now every written file is checksummed by default this commit
also verifies the checksum for files during recovery and restore if
applicable.

Closes #5924
Remove BufferedChecksumIndexOutput.java
Added test that corrupts files on the network layer
Remove unused class
Apply changed after upgrade to Lucene 4.9 & add more javadocs