Opaque, error-handling and ROLLBACK cleanup.

- application supplied `opaque` value is 16-bit.
- helper functions to manipulate opaque parts.
- CloseStream() to delete stream object from feed.
- check for active stream for UPR_CLOSESTREAM,
  UPR_FLUSH, UPR_SNAPSHOT, UPR_MUTATION, UPR_DELETION,
  UPR_EXPIRATION
- avoid automatic ROLLBACK handling by gomemcached, it
  now delegated to application. The stream is considered
  inactive and delete it from feed.

Change-Id: Ia527e2d14851b4d588a763ae683a87305ecd5478