Implementing Server-side Stores for Write Operations

This review is getting quite big, so I'm going to cut it off here at a working point. Code is to be merged into a separate branch (not voldemort/master)

Includes:
- PUT and DELETE workflow working through VeniceStore
- Included configuration by store, rather than by node
- Cleaned up code as per Sid's and Felix's comments

Not Included:
- Offset Management
- Throttling
Oops! This comment was left behind after I removed the "multiple threads per partition" logic. You're right that it should only be p.