[Docs] Fix incorrect eclipse m2e markdown syntax

Closes #7546.
[docs] fix typo in language analyzer docs
Core: Remove DocSetCache.

This class was unused.

Close #7582
Internal: deduplicate useful headers that get copied from REST to transport requests

The useful headers are now stored into a `Set` instead of an array so we can easily deduplicate them. A set is also returned instead of an array by the `usefulHeaders` static getter.

Relates to #6513

Closes #7590
Refactored TransportMessage context

Removed CHM in favour of an OpenHashMap and synchronized accessor/mutator methods. Also, the context is now lazily inititialied (just like we do with the headers)
Indexed scripts: make sure headers are handed over to internal requests and streamline versioning support

The get, put and delete indexed script apis map to get, index and delete api and internally create those corresponding requests. We need to make sure that the original headers are handed over to the new request by passing the original request in the constructor when creating the new one.

Also streamlined the support for version and version_type in the REST layer since the parameters were not consistently parsed and set to the internal java API requests.

Modified the REST delete template and delete script actions to make use of a client instead of using the `ScriptService` directly.

Closes #7569
Docs: Fixed typo in windows-as-a-service
Internal: Upgrade Guava to 18.0.

17.0 and earlier versions were affected by the following bug
https://code.google.com/p/guava-libraries/issues/detail?id=1761
which caused caches that are configured with weights that are greater than
32GB to actually be unbounded. This is now fixed.

Relates to #6268
Close #7593
Term Vectors: Support for artificial documents

This adds the ability to the Term Vector API to generate term vectors for
artifical documents, that is for documents not present in the index. Following
a similar syntax to the Percolator API, a new 'doc' parameter is used, instead
of '_id', that specifies the document of interest. The parameters '_index' and
'_type' determine the mapping and therefore analyzers to apply to each value
field.

Closes #7530
TransportClient: Mark transport client as such when instantiating

This allows plugins to load/inject specific classes, when the client started
is a transport client (compared to being a node client).

Closes #7552