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:

- Remove 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
- Apply @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)