[SPARK-13809][SQL] State store for streaming aggregations

## What changes were proposed in this pull request?

In this PR, I am implementing a new abstraction for management of streaming state data - State Store. It is a key-value store for persisting running aggregates for aggregate operations in streaming dataframes. The motivation and design is discussed here.

https://docs.google.com/document/d/1-ncawFx8JS5Zyfq1HAEGBx56RDet9wfVp_hDM8ZL254/edit#

## How was this patch tested?
- [x] Unit tests
- [x] Cluster tests

**Coverage from unit tests**

<img width="952" alt="screen shot 2016-03-21 at 3 09 40 pm" src="https://cloud.githubusercontent.com/assets/663212/13935872/fdc8ba86-ef76-11e5-93e8-9fa310472c7b.png">

## TODO
- [x] Fix updates() iterator to avoid duplicate updates for same key
- [x] Use Coordinator in ContinuousQueryManager
- [x] Plugging in hadoop conf and other confs
- [x] Unit tests
  - [x] StateStore object lifecycle and methods
  - [x] StateStoreCoordinator communication and logic
  - [x] StateStoreRDD fault-tolerance
  - [x] StateStoreRDD preferred location using StateStoreCoordinator
- [ ] Cluster tests
  - [ ] Whether preferred locations are set correctly
  - [ ] Whether recovery works correctly with distributed storage
  - [x] Basic performance tests
- [x] Docs

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #11645 from tdas/state-store.