SCTP: Reduce object allocation overhead and fix receive buffer allocation

There are a couple of changes here all related to making the SCTP transport less garbage-heavy:
* 7147e57 removes the SctpNotificationEvent and just passes along the JDK NIO Notification, as passing the Notification and always null inside a wrapped object seemed a little bit superfluous
* 75ece85 applies @trustin's changes to receive buffer allocation to SCTP transport, and also makes the SCTP transport use the configured buffer allocator rather than always creating a direct buffer (which seems like a bug)