[TEST] Expose ability to provide http headers when sending requests in our REST tests

ElasticsearchRestTests has now a `restClientSettings` method that can be overriden to provide headers as settings (similarly to what we do with transport client). Those headers will be sent together with every REST requests within the tests.

Closes #7710
Internal: make sure that original headers are used when executing search as part of put warmer

Closes #7711
Test: ZenUnicastDiscoveryTests.testNormalClusterForming should start unicast hosts first

The test starts a cluster with random nodes as unicast hosts but *doesn't* use min_master_nodes. If the unicast hosts are started last, nodes may elect themselves as master as they do not have mechanism yet to share information.
function score: fix cast in Gaussian decay function

Also fix the test
FunctionScoreTests#simpleWeightedFunctionsTestWithRandomWeightsAndRandomCombineMode
which sometimes failed due to rounding issues. Make sure
only floats are returned as scores to assure ratio of
expected and returned score is 1.0f.
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
Replaces #7588
Only set `breaker` when stats are retrieved

When communicating with 1.3 and earlier nodes, it's possible that the
field data breaker info is not sent at all. When this happens, we should
leave the `breaker` variable as-is (unset) instead of creating an
AllCircuitBreakerStats object with a null fd breaker and fake request &
parent breakers.