[TESTS] Add explicit mappings to IndexAliasesTests.testSearchingFilteringAliasesSingleIndex

This makes sure that all shards know about the `_uid` field.
Aggregations: Adds ability to sort on multiple criteria

The terms aggregation can now support sorting on multiple criteria by replacing the sort object with an array or sort object whose order signifies the priority of the sort. The existing syntax for sorting on a single criteria also still works.

Contributes to #6917
_timestamp: enable mapper properties merging

Updates on the _timestamp field were silently ignored.
Now _timestamp undergoes the same merge as regular
fields. This includes exceptions if a property cannot
be changed.
"path" and "default" cannot be changed.

closes #5772
closes #6958
closes #7614
partially fixes #777
[TEST] added @AwaitsFix to failing StringTermsTests while I work on a fix