explain score: fix explanation streaming

Complex explanations were always read as Explanations. Depending
on if the response was streamed or not the explanation was
therefore generated by a ComplexExplanation or by a regular
Explanation.

closes #7257
bool query: parser should return match_all in case there are no clauses

This also fixes has_parent filters with a nested empty bool filter
(see test SimpleChildQuerySearchTests#test6722, the test should actually expect
either 0 results when searching for has_parent "test" or one result when
search for has_parent "foo")

closes #7240
closes #7347
Fielddata: Remove soft/resident caches.

These caches have no advantage compared to the default node cache. Additionally,
the soft cache makes use of soft references which make fielddata loading quite
unpredictable in addition to pushing more pressure on the garbage collector.

The `none` cache is still there because of tests. There is no other good
reason to use it.

LongFieldDataBenchmark has been removed because the refactoring exposed a
compilation error in this class, which seems to not having been working for a
long time. In addition it's not as much useful now that we are progressively
moving more fields to doc values.

Close #7443
Add translog checksums

Switches TranslogStreams to check a header in the file to determine the
translog format, delegating to the version-specific stream.

Version 1 of the translog format writes a header using Lucene's
CodecUtil at the beginning of the file and appends a checksum for each
translog operation written.

Also refactors much of the translog operations, such as merging
.hasNext() and .next() in FsChannelSnapshot

Relates to #6554
Indices API: Added GET Index API

Returns information about settings, aliases, warmers, and mappings. Basically returns the IndexMetadata.

Closes #4069
Implemented review comments
more review comments
Created proper Java API to access IndexMetadata
bug fixes
bug fixes to make all tests pass