Core: trigger refresh when the live version map is using too much RAM

When refresh_interval is long or disabled, and indexing rate is high,
it's possible for live version map to use non-trivial amounts of RAM.
With this change we now trigger a refresh in such cases to clear the
version map so we don't use unbounded RAM.

Closes #6443
Translog: change flush_threshold_ops to unlimited by default, so we only flush by byte size by default

If you are indexing tiny documents then the previous default (5000)
was too low, causing excessive fsyncs with high indexing rates.  With
this change we now only flush by byte size (200 MB) by default for
better indexing performance for tiny documents.

Closes #6783
Add assertBusy helper test method
We use awaitBusy in our tests, the problem is that we have to check if it awaited or not, and then try and keep around somehow more info around why the predicate failed and a timeout happened.
The idea of assertBusy is to allow to simply write "regular" test code, and if the test code trips, it will busy wait till a timeout. This allows us to keep around the assertion information and properly throw it for information that is inherently kept in the failure itself.
[TEST] move tribe tests to use assertBusy
Add missing pre built analysis components
closes #6770
Remove intern calls on FieldMapper#Names
remove internal callas on FieldMappers#Names, we properly reuse FieldMapper, so there is no need to try and call intern in order to reuse the names. This can be heavy with many fields and continuous mapping parsing.
closes #6747
[API SPEC] Update indicies.stats group param
add more options to many mappings test
[ENGINE] Prevent NPE if engine is closed while version map is checked

We check if the version map needs to be refreshed after we released
the readlock which can cause the the engine being closed before we
read the value from the volatile `indexWriter` field which can cause an
NPE on the indexing thread. This commit also fixes a potential uncaught
exception if the refresh failed due to the engine being already closed.

Relates to #6443
Closes #6786