Provide a K-V Store API in LS-Core

@suyograo let me know if I got this right :)

#### We want

Generic K-V store API usable by plugins and in LS core, backed by a configurable store (in step 1 this should be ES).

#### How about this?

* Configure state store in `logstash.yml` (es host etc.)
* If not configured use temp file that is only valid for the lifetime of the LS process
* Provide Java static API that allows you to get an isolated store by name like `LsStateStore.getOrCreate("NameOfStatestore")` that will then be backed by the configured backend from in the `logstash.yml`
* Have `LsStateStore` provide `get()`, `set()`, `delete()` for any `String` key and `String` value

How does that sound?