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
[Engine] checkVersionMapRefresh shouldn't use indexWriter.getConfig(). We run it out of lock, the indexWriter may be closed..

Relates to #6443, #6786

Closes #6794
Update preference.asciidoc

Clarify that `preference` is a query string parameter only
and provide an example.
[Code] Control whether MapperService docMapper iterator should contain DEFAULT_MAPPING

At the moment one can iterate the MapperService to go through all document mappers. This includes the document mapper of DEFAULT_MAPPING, which may be surprising and lead to unintended results. This commit removes the Iterable implementation and add a docMappers method that asks the caller to make an explicit choice

Closes #6793
Store: Before removing shard physically from disk verify that another node in the cluster actually holds an active shard copy.

Closes #6692
Improve Settings#get lookup for camel case support
Today, if we miss on a get on setting, we try its camel case version. The assumption is that in our code, we never use camel case to lookup a setting, but we want to support camel case if the user provided one.
This can be expensive (#toCamelCase) when the get on the setting is done in a tight call, which is evident when running the allocation deciders as part of the reroute logic.
Instead of doing the camel case check on get, prepare an additional map that includes all the settings that are provided as came case, and try and lookup from it if needed.
closes #6765
Add v1.2.3 to Version.java
Fix typo in Hunspell logging
Docs: Update plugins.asciidoc

Closes #6683
[STORE]: Make use of Lucene build-in checksums

Since Lucene version 4.8 each file has a checksum written as it's
footer. We used to calculate the checksums for all files transparently
on the filesystem layer (Directory / Store) which is now not necessary
anymore. This commit makes use of the new checksums in a backwards
compatible way such that files written with the old checksum mechanism
are still compared against the corresponding Alder32 checksum while
newer files are compared against the Lucene build in CRC32 checksum.

Since now every written file is checksummed by default this commit
also verifies the checksum for files during recovery and restore if
applicable.

Closes #5924
mute test
Fix snapshot tests

cleanup Store part where metadata is read ie. if segments.gen is not
present it's no reason to fail the meta request.
Added logic that removes partially written files during restore.