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
Test: Allow global test cluster to have configurable settings source

This allows to reuse the global test cluster with specific configurations,
which is useful in plugins.
FunctionScore: Refactor RandomScoreFunction to be consistent, and return values in rang [0.0, 1.0]

RandomScoreFunction previously relied on the order the documents were
iterated in from Lucene. This caused changes in ordering, with the same
seed, if documents moved to different segments. With this change, a
murmur32 hash of the _uid for each document is used as the "random"
value. Also, the hash is adjusted so as to only return values between
0.0 and 1.0 to enable easier manipulation to fit into users' scoring
models.

closes #6907, #7446