[TEST] fix issue clearing fielddata breaker introduced in 6950c38a0436ec937797f01fba8d7d95e6d6225f
[TEST] Explicitly clean up actions to be intercepted in IndicesRequestTests before asserting on collected requests

This helps making sure that no further requests are collected once we start asserting on them
[Internal] discovery.id.seed is ignored

Closes #7439, Closes #7437
Stats: add segments.index_writer_max_memory to see index writer's max RAM usage before buffered documents must be written to a new segment

Closes #7438

Closes #7440
[TESTS] Temporary disable field data cache randomization.
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
[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