Core: don't close/reopen IndexWriter when changing RAM buffer size

Today we close/reopen IW when we change the RAM buffer but that's
costly because it means the next NRT reader is a full reopen.  The RAM
buffer size setting is a live one in IndexWriter, even if there are no
buffered docs in RAM when you call it.

Separately it would be nice if Lucene let you manage a "reader pool"
that could outlive individual IW instances ...

Closes #6856
Internal: make transport action name available in TransportAction base class

Each transport action is associated with at least an action name, which is the action name that gets serialized together with the request and identifies what to do with the request itself. Also, the action name is the name of the registered transport handler that handles incoming request for the transport action.

This commit makes the action name available in a generic manner in the TransportAction base class, so that it can be used when needed by subclasses, or in the base class for instance for action filtering.

Closes #6860
[UPDATE] Handle document missing exception on retry correctly.

Throwables thrown on update retries are now caught and handled via
the provided callback. This commit also contains an integration test
demonstrating the bug and validating the fix.

Closes #6355
Closes #6724
[TESTS] Add ElasticsearchSingleNodeTest.

This test makes it easy to create a lightweight node (no http, indices stored
in RAM, ...) whose main purpose is to get an instance of the Guice injector
for unit tests.

This should help not have to update lots of unit tests when we add a new
Guice dependency.
Added [1.4.0] version
Scripting: Add script engine for lucene expressions.

These are javascript expressions, which can only access numeric
fielddata, parameters, and _score. They can only be used for searches (not document updates).

closes #6818
[TEST] Enforce provided settings in test base classes

Enviorment variables might override the tests settings even if
they are explicitly set. Other base classes like InternalTestCluster
also specify `config.ignore_system_properties: true` to ensure `what
we set is what we get`
Test: make test less evil so it doesn't take so long
[TEST] Enforce that the node from ElasticsearchSingleNodeTest is alone in its cluster.

In particular, this removes the ability to create new node and adds the ability
to get a reference to the single node that the cluster is made of.
Docs: explain that SerialMergeScheduler just maps to CMS for back compat

Closes #6878
Mappings: Add transform to document before index.

Closes #6566
[FIX] normalize serialization of ScriptType in UpdateRequest