Merge pull request #4675 from spenceralger/update_spec_doc_urls

Updated the documentation urls in the rest api spec.
Multi data path config can cause a shard to be perceived as corrupted
Multi data path config support writes a file to a data location based on the available size (by default). There is a Lucene file called segments.gen that has the same name, and only in that case, we need to make sure we alway write it to the same data location, otherwise, the index will have multiple segments.gen files, and the shard can seem to be corrupted.

The message if this case happens is that segments_xxx file was not found, in which case, a find for segments.gen can yield multiple files. Deleting the segments.gen files will cause the shard to recover properly (as its an extra protection layer to resolve the segments header by Lucene)

Make sure the segments.gen file is writtne to the same directory every time
fixes #4674
Open files with encoding='utf-8' in release script
Removed needless TestRiver used in RiverTests

DummyRiver can be used instead, no need for TestRiver
Add 0.90.10 to master
Add new option `min_doc_count` to terms and histogram aggregations.

`min_doc_count` is the minimum number of hits that a term or histogram key
should match in order to appear in the response.

`min_doc_count=0` replaces `compute_empty_buckets` for histograms and will
behave exactly like facets' `all_terms=true` for terms aggregations.

Close #4578