[root mappers] fix conflict when updating mapping with _all disabled

_all reports a conflict since #7377. However, it was not checked if _all
was actually configured in the updated mapping. Therefore whenever _all
was disabled a mapping could not be updated unless _all was again added to the
updated mapping.
Also, add enabled setting to mapping always whenever enabled was set explicitely.

closes #8423
closes #8426
Parser throws NullPointerException when Filter aggregation clause is empty.
Added Junit test that recreates the error and fixed FilterParser to default to using a MatchAllDocsFilter if the requested filter clause is left empty.
Also added fix and test for the Filters (with an "s") aggregation.

Closes #8438
[STORE] Remove `memory`/ `ram` store

The RAM store is discuraged for production usage anyway and
we don't test it in our randomized infrastructure. This commit
removes it for `2.0`
Revert "[TEST] use logger level from test class annotation also in external nodes"

This reverts commit 4604a68bef9c54f5db812609157b18788279012d.
Add missing await into an interruptable block
[TEST] Delete index in test to release file handles
[INDEX] Add before/after indexDeleted callbacks to IndicesLifecycle

In order to implement #8551 correctly without causing problems of relocating
shards we need to be informed if an index is actually deleted. This commit adds
more callbacks to the listener and makes deleteIndex a dedicated method on IndicesService
[CORE] Free pending search contexts if index is closed

Today we hold on to search context reference if they are not cleaned
up for a while until a reaper thread trashes them if they timed out.
This commit removes all pending contexts once the index is closed to release
resources and filehandles immediatly once the index is closed.