Use UTF-8 as string encoding
Add some finals where appropriate
Use EnumSet rather than static mutable arrays

ClusterBlockLevel uses arrays but should use EnumSets instead
Remove SoftReferences from StreamInput/StreamOutput

We try to reuse character arrays and UTF8 writers with softreferences.
SoftReferences have negative impact on GC and should be avoided in
general. Yet in this case it can simply replaced with a per-stream
Bytes/CharsRef that is thread local and has the same lifetime as the
stream.
[DOCS] Renamed the "cat" chapters to be more searchable
Allow 0 as a valid external version

Until now all version types have officially required the version to be a positive long number. Despite of this has being documented, ES versions <=1.0 did not enforce it when using the `external` version type. As a result people have succesfully indexed documents with 0 as a version. In 1.1. we introduced validation checks on incoming version values and causing indexing request to fail if the version was set to 0. While this is strictly speaking OK, we effectively have a situation where data already indexed does not match the version invariant.

To be lenient and adhere to spirit of our data backward compatibility policy, we have decided to allow 0 as a valid external version type. This is somewhat complicated as 0 is also the internal value of `MATCH_ANY`, which indicates requests should succeed regardles off the current doc version. To keep things simple, this commit changes the internal value of `MATCH_ANY` to `-3` for all version types.

Since we're doing this in a minor release (and because versions are stored in the transaction log), the default `internal` version type still accepts 0 as a `MATCH_ANY` value. This is not a problem for other version types as `MATCH_ANY` doesn't make sense in that context.

Closes #5662
Add Partial snapshot state

Currently even if some shards of the snapshot are not snapshotted successfully, the snapshot is still marked as "SUCCESS". Users may miss the fact the there are shard failures present in the snapshot and think that snapshot was completed. This change adds a new snapshot state "PARTIAL" that provides a quick indication that the snapshot was only partially successful.

Closes #5792