Internal: made original indices optional for broadcast delete and delete by query shard requests

Shard requests like broadcast delete and delete by query, that needs to be executed on primary and all replicas, get read and written out to the transport on the same node. That means that if we add some field version checks are not enough to maintain bw comp since a newer node that holds the primary might receive the request from an older node, that didn't provide the field. Yet, when writing the request out again to a newer node that holds the replica, we do try and serialize the field although it's missing. The newer fields just needs to be set to optional in these cases, in addition to the version checks.

Re-enabled testDeleteByQuery and testDeleteRoutingRequired bw comp tests since this was the cause of their failures.

Closes #7406
[Tests] NoMasterNodeTests make timeout checks less sensitive

Also remove catching of MasterNotDiscoveredException in bulk operation it is only set on a per item basis
[Tests] testNodeVersionIsUpdated stopped but didn't close it's node
[TEST] use more verbose assertion in IndicesRequestTests
[TEST] Agg may not be a instance of StringTerms, but UnmappedTerms, so use common Terms class instead
Nested: If the `_type` field isn't indexed nested docs must be filtered out.
[DOCS] Document NOT as a simple_query_string flag

Fixes #7426
Test: Fixed pluggable transport module test to support transportclient

Also made sure, that only a change of requests is tested for and not
an initial value, which might not be set in case of a node client.
Tests: Improve test coverage.

Close #7428
Context suggester: infinite loop in GeolocationContextMapping

Close #7433
[TESTS] Temporarily disable FileBasedMappingsTests.
Test: Fixed the regex in cat.indices/10_basic.yaml

And renamed the file from .yml to .yaml
[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