Add new Sequencer interfaces and service

Add new interfaces for sequencers that use and expose the JCR API. Also add a sequencing service (used internally by each repository instance) that encapsulates the logic of monitoring for the changes, determining which (if any) sequencers should be invoked, the invocation of the appropriate sequencers, and the saving of the changes made by each sequencer. Note that this new service should use a (JMS) durable queue for the sequencing events.