[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
Make .zip and .tar.gz release artifacts contain same files.

This commit changes the build to include .exe and sigar/.dll files in
both the zip and tar artifacts.

Closes #2793
[CORE] Unify search context cleanup

Today there are two different ways to cleanup search contexts which can
potentially lead to double releasing of a context. This commit unifies
the methods and prevents double closing.

Closes #7625
[Stats] update action returns before updating stats for `NONE` operations

We keep around a noop stats indicating how many update operations ended up not updating the document (typically because it didn't change). However, the TransportUpdateAction update that counter only after returning the result. This can throw off stats check which are done immediately after, potentially causing test failures.

Closes #7639
[TEST] Don't print BWC test path - it's different on every machine
[TESTS]: create directory for heapdumps if not exists

Before the heapdump was either written in a file with the
directory name if the heapdump path ended without / or
not written at all if the path ended with /

closes #7645
Revert "[TEST] added @AwaitsFix to failing StringTermsTests while I work on a fix"

This reverts commit 18a713a2ae896e72c5821bf3f6d239ce634d1f52.
Revert "Aggregations: Adds ability to sort on multiple criteria"

This reverts commit bfedd11ffa4c48be0afd91793f491b0a8ff0ef8d.
[STORE] Write Snapshots directly to the blobstore stream

Today we serialize the snashot metadata to a byte array and then copy
the byte array to a stream. Instead this commit moves the serialization
directly to the target stream without the intermediate representation.

Closes #7637
Quiet BWC analysis test case for now.

See https://issues.apache.org/jira/browse/LUCENE-5927.
CORS: Disable by default

In order to deliver a more secure out-of-the-box configuration this commit
disables cross-origin resource sharing by default.

Closes #7151