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