Throw ESIA if required field 'patterns' is not present in filter config.

If 'pattern_capture' tokenfilter is create / mapped without a 'patterns'
settings we now throw an exception since this is a misconfiguration and
likely due to the similar settings on related token filters.

Closes #3808
Add more anchor links to documentation

Related to #3679
Uniquify anchor links to fix asciidoc/docbook generation
Calling `GET _template` gave a 500 error back.

Also:
We always return a valid json back
If there are no templates define we return 200 OK ( as opposed to a request for a specific template id which returns 404)

Closes #3812
Improved index template tests, among other to test for getting all templates by not specifying a name.
StreamTests only work on 64Bit JVMs

32bit JVMs have different buffer grow behavior. We simply ingore this
since it's a lucene feature that is tested in the upstream project.
Add more debug info to testcase
s/Exception/Throwable
fix term vector api for missing payloads
Allow dynamic templates with match_mapping_type but no path_match or match.

As long as we have some selector we're good.

Closes #3814
Make testcase more reproducible
Uniquify more anchor links to fix asciidoc
[snapshot/restore] Add ability to register snapshot repositories
[snapshot/restore] Add support for Repository Instantiation
[snapshot/restore] Add snapshot creation mechanism
[snapshot/restore] Rename snapshot endpoint to repository
[snapshot/restore] Add snapshot status update mechanism
[snapshot/restore] Add snapshot simple end-to-end snapshotting test
[snapshot/restore] Add basic file system store implementation
[snapshot/restore] Add file snapshotting