Before this commit, the return value from write was interpreted as the
data being fully written and ready to be released via releaseData().
This is not true for WebSocketSession implementations where a true
return value simply means the message was sent with the full payload
but releas is not appropriate until a send confirmation.
Technically not an issue since WebSocketSession's extending this do
not use pooled buffers. Nevertheless this commit refines the semantics
of write, removes the releaseData() method, and makes sub-classes
responsible for releasing the buffer when fully written (and they
know best when that is). As a bonus currentData is now private.
Issue: SPR-16207
01a82b5291