add functionality to unload objects

Add a way to unload game objects from the live object cache (in the
persistence layer) after request processing. This is required e.g.
when moving players from one GS to another.
optional callback for post-request persistence operations

Provide a way to specify a function to be called when post-request
persistence operations have finished (necessary for player moves
between game servers).
Merge pull request #5 from yelworc/pers-unload

unload objects from persistence layer cache