Docs: Added link to clarify meaning of filtering in fielddata context
Core: use Java's built-in ConcurrentHashMap

It's risky to have our own snapshot of Java 8's ConcurrentHashMap:
unless we keep the sources in sync over time (and OpenJDK's version
had already diverged), then we won't get bug/performance fixes.  Users
can choose to upgrade to Java 8 to see the improvements of CHM.

Closes #7392

Closes #7296
_all: report conflict on merge and throw exception on doc_values

- _all field was never merged when mapping was updated and no conflict reported
- _all accepted doc_values format although it is always tokenized

relates to #777
closes #7377
[ENGINE] Wait until engine is started up when acquireing searcher

Today we have a small window where a searcher can be acquired but the
engine is in the state of starting up. This causes a NPE triggering a
shard failure if we are fast enough. This commit fixes this situation
gracefully.

Closes #7455
Docs: typo?

Closes #7458
[SNAPSHOT] Add BWC layer to .si / segments_N hashing

Due to additional safety added in #7351 we compute now a strong hash for
.si and segments_N files which are compared during snapshot / restore.
Old snapshots don't have this hash which can cause unnecessary copying
of large amount of data. This commit adds the ability to fetch this
hash from the blob store if needed.

Closes #7434
Test: mute test until we know what is going on
[Test] rewrite testNoMasterActions to use latest tooling

The test's timeout checks were thrown off by a client created randomly (when the timer was running).

Closes #7432
Tests: Fix FileBasedMappingsTests by using the mappings API instead of field mappings.
Tests: Temporarily ignore SnapshotBackwardsCompatibilityTest
Internal: Add LZF safe encoder in LZFCompressor

Selecting the safe encoder fixes a 64bit JVM crash on big-endian architectures with
LZF UnsafeChunkEncoderBE.

closes #7466
Revert part of change in #7466 to fix issue because encoder is not threadsafe so cannot be shared
Internal: Add all unsafe variants of LZF compress library functions to forbidden APIs.

The "optimized" encoders/decoders have been unreliable and error prone.
Also, fix LZFCompressor.compress to use LZFEncoder.safeEncode, which
creates a new safe encoder, instead of using a shared encoder (which
is not threadsafe).

closes #7468
[Test] testLargeClusterStatePublishing - bound max shard no to number of nodes and set replica count to 0

ensureGreen sometimes times out due to too many shards and GC kicking in