Add configurable batch-size to AbstractStoreConfiguration

We should add a configurable batch-size parameter to the AbstractStoreConfiguration so that the number of batch writes to a store can be controlled on a per store level.

This is necessary as some store implementations may encounter problems if batches are too large, e.g. JDBC https://stackoverflow.com/questions/3536103/mysql-how-many-rows-can-i-insert-in-one-single-insert-statement/27855325#27855325