Added the necessary database code to convert the contents of event
queues into actual event records. The changes include:
* a new table, events.pending_events, which is automatically filled by
a trigger when events are inserted into queue tables,
* the game.events.batchSize constant which defines the maximal amount
of events to process in a single transaction,
* the events.eq_process() stored procedure, which processes the events.
In addition, the "hstore" extension was added as it is the easiest way
to convert events from the queues' table model to the store's
meta-model.
Because the events management functions are definitely going to be quite
big, I thought it would be better to split them into multiple SQL files.
For now, there's a file for event definition functions and one for
functions that manipulate priority overrides. The old file containing
the old code stays around for now.
