Randomize CMS settings in index template

This commit adds randomization for:
 * `index.merge.scheduler.max_thread_count`
 * `index.merge.scheduler.max_merge_count`

This commit also moves to use
EsExecutors#boundedNumberOfProcessors(Settings) to default
configure the default `max_thread_count` for better reproducibility

Closes #6194
Check that a plugin is Lucene compatible with the current running node using `lucene` property in `es-plugin.properties` file.

* If plugin does not provide `lucene` property, we consider that the plugin is compatible.
* If plugin provides `lucene` property, we try to load related Enum org.apache.lucene.util.Version. If this fails, it means that the node is too "old" comparing to the Lucene version the plugin was built for.
* We compare then two first digits of current node lucene version against two first digits of plugin Lucene version. If not equal, it means that the plugin is too "old" for the current node.

Plugin developers who wants to launch plugin check only have to add a `lucene` property in `es-plugin.properties` file. If you are using maven to build your plugin, you can do it like this:

In `pom.xml`:

```xml
    <properties>
        <lucene.version>4.6.0</lucene.version>
    </properties>

    <build>
        <resources>
            <resource>
                <directory>src/main/resources</directory>
                <filtering>true</filtering>
            </resource>
        </resources>
    </build>
```

In `es-plugin.properties`, add:

```properties
lucene=${lucene.version}
```

BTW, if you don't already have it, you can add the plugin version as well:

```properties
version=${project.version}
```

You can disable that check using `plugins.check_lucene: false`.
Add simple example to XContentParser how to obtain an instance of it
RecoveryID should not be a per JVM but per Node

Today the RecovyerID is taken from a static atomic long which
is essentially a per JVM ID. We run the tests within the same
JVM and that means we don't really simulate what happens in
production environments. Instead we should use a per node generated
ID.
Remove dead code
[TEST] Do not filter gz files
added install instruction with apt

Closes #6206
Use UTF-8 as string encoding
Add some finals where appropriate
Use EnumSet rather than static mutable arrays

ClusterBlockLevel uses arrays but should use EnumSets instead
Remove SoftReferences from StreamInput/StreamOutput

We try to reuse character arrays and UTF8 writers with softreferences.
SoftReferences have negative impact on GC and should be avoided in
general. Yet in this case it can simply replaced with a per-stream
Bytes/CharsRef that is thread local and has the same lifetime as the
stream.
[DOCS] Renamed the "cat" chapters to be more searchable
Allow 0 as a valid external version

Until now all version types have officially required the version to be a positive long number. Despite of this has being documented, ES versions <=1.0 did not enforce it when using the `external` version type. As a result people have succesfully indexed documents with 0 as a version. In 1.1. we introduced validation checks on incoming version values and causing indexing request to fail if the version was set to 0. While this is strictly speaking OK, we effectively have a situation where data already indexed does not match the version invariant.

To be lenient and adhere to spirit of our data backward compatibility policy, we have decided to allow 0 as a valid external version type. This is somewhat complicated as 0 is also the internal value of `MATCH_ANY`, which indicates requests should succeed regardles off the current doc version. To keep things simple, this commit changes the internal value of `MATCH_ANY` to `-3` for all version types.

Since we're doing this in a minor release (and because versions are stored in the transaction log), the default `internal` version type still accepts 0 as a `MATCH_ANY` value. This is not a problem for other version types as `MATCH_ANY` doesn't make sense in that context.

Closes #5662
Add Partial snapshot state

Currently even if some shards of the snapshot are not snapshotted successfully, the snapshot is still marked as "SUCCESS". Users may miss the fact the there are shard failures present in the snapshot and think that snapshot was completed. This change adds a new snapshot state "PARTIAL" that provides a quick indication that the snapshot was only partially successful.

Closes #5792
Switch to shared thread pool for all snapshot repositories

 Closes #6181
[TESTS] Ignore SnapshotMissingException in snapshotWithStuckNodeTest

The retry mechanism in the transport layer might cause the delete snapshot request to be executed twice if the cluster master is closed while the request is executed. First time delete snapshot request is getting successfully executed on the old master and then it is retried on the newly elected master. When the new master tries to delete the snapshot - the snapshot no longer exists (since it was successfully deleted by the old master) and SnapshotMissingException is returned.
More Like This Query: Added searching for multiple items.

The syntax to specify one or more items is the same as for the Multi GET API.
If only one document is specified, the results returned are the same as when
using the More Like This API.

Relates #4075 Closes #5857
[TEST] Stabelize BenchmarkIntegrationTest#testAbortBenchmark
DeleteRequest.version was not initialized to `Versions.MATCH_ANY`
Translog: remove unneeded Versions.readVersion & Versions.writeVersion

These calls were introduced in pr #6149 as a backward compatibility layer for the previous value of `Versions.MATCH_ANY`. This is not needed as the translog never contains these values. On top of that, the calls are not effective as the stream the translog used is effectively not versioned (versioining is done on an item by item basis)