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.