Refactoring DataStore API.

This is the first of an expected number of commit related to changing
the DataStore API to work well with all kinds of databases (relational
and none-relational).
Moving protocol impl to protocol module.
Removed removeChannel from DataStore interface.
Refactored DefaultSimplePushServerTest to accomodate for usage of different datastore implementations.
Moving DefaultChannel to server-api
Adding JpaSimplePushServerTest
Removing unused imports.
Adding javadoc to MutableChannel
Adding default tokenKey and removing sysout.
Merge branch 'danbev-AGPUSH-381-datastore'