Aggregations: Fix reducing of range aggregations.

Under some rare circumstances:
 - local transport,
 - the range aggregation has both a parent and a child aggregation,
 - the range aggregation got no documents on one shard or more and several
   documents on one shard or more.
the range aggregation could return incorrect counts and sub aggregations.

The root cause is that since the reduce happens in-place and since the range
aggregation uses the same instance for all sub-aggregation in case of an
empty bucket, sometimes non-empty buckets would have been reduced into this
shared instance.

In order to avoid similar bugs in the future, aggregations have been updated
to return a new instance when reducing instead of doing it in-place.

Close #6435
Aggregations: Remove `ordinals` execution hint.

This was how terms aggregations managed to not be too slow initially by caching
reads into the terms dictionary using ordinals. However, this doesn't behave
nicely on high-cardinality fields since the reads into the terms dict are
random and this execution mode loads all unique terms into memory.

The `global_ordinals` execution mode (default since 1.2) is expected to be
better in all cases.

Close #6499
Fielddata: Don't expose hashes anymore.

Our field data currently exposes hashes of the bytes values. That takes roughly
4 bytes per unique value, which is definitely not negligible on high-cardinality
fields.

These hashes have been used for 3 different purposes:
 - term-based aggregations,
 - parent/child queries,
 - the percolator _id -> Query cache.

Both aggregations and parent/child queries have been moved to ordinals which
provide a greater speedup and lower memory usage. In the case of the percolator
it is used in conjunction with HashedBytesRef to not recompute the hash value
when getting resolving a query given its ID. However, removing this has no
impact on PercolatorStressBenchmark.

Close #6500
[TEST] Stabelize test - wait for yellow to ensure all primaries are allocated
Better default size for global index -> alias map
The alias -> (index -> alias) map, specifically the index -> alias one, typically just hold one entry, yet we eagerly initialize it to the number of indices. When there are many indices, each with many aliases, this is a very large overhead per alias...
closes #6504
[DOCS] Fixd typo in http.asciidoc
[TEST] consistently omit norms in test otherwise scoring will be dependent on merges etc.
[TEST] Force flush even if not needed to ensure successful shards is greater than 0
[TEST] Add basic Backwards Compatibility Tests

This commit add a basic infrastructure as well as primitive tests
to ensure version backwards compatibility between the current
development trunk and an arbitrary previous version. The compatibility
tests are simple unit tests derived from a base class that starts
and manages nodes from a provided elasticsearch release package.

Use the following commandline executes all backwards compatiblity tests
in isolation:

```
mvn test -Dtests.bwc=true -Dtests.bwc.version=1.2.1 -Dtests.class=org.elasticsearch.bwcompat.*
```

These tests run basic checks like rolling upgrades and
routing/searching/get etc. against the specified version. The version
must be present in the `./backwards` folder as
`./backwards/elasticsearch-x.y.z`
[TEST] Add test for accessing _score in scripts
[TEST] Fix test cluster naming

This commit renames `TestCluster` -> `InternalTestCluster` and
`ImmutableTestCluster` to `TestCluster` for consistency. This also
makes `ExternalTestCluster` and `InternalTestCluster` consistent
with respect to their execution environment.

Closes #6510
[DOCS] Fixed is/if typo in Api Conventions doc
[TEST] wait for ongoing recoveries to finish. Flush fails on shards otherwise.
[TEST] verify all threads created by node and client have the node name
closes #6516
Use ordinals for comparison in GlobalOrdinalsStringTermsAggregator.buildAggregation. Closes #6518
[Benchmark] Fix TermsAggregationSearchBenchmark: The `ordinals` execution mode doesn't exist anymore.
lightweight minimum master node recovery
don't perform full recovery when minimum master nodes are not met, keep the state around and use it once elected as master
make no master lock an instance var so it can be configured
add rejoin on master gone flag, defaults to false
defaults to false since there is still work left to properly make it work
Make noMasterBlock configurable and added simple test that shows reads do execute (partially) when m_m_n isn't met