Don't discard data on io.ErrShortBuffer

Currently if the user passes a buffer to read into that is shorter then
the current message we lose the buffer. This PR changes the behavior to
instead keep what's in the reassemblyQueue until it can be read.

Relates to pion/webrtc#2712

Set rwnd from InitChunk (#364)

The rwnd has not been initialized from InitChunk
then payload data will be hold in the pending
queue until first SACK is received.

Return 0 written bytes on write error

Return 0 written bytes on write error

Increment missing indications from SACK

Increment missing indication for reported
missing chunks instead of all inflight chunks.

set close state before unregisterStream

Unblock handshake ch on close

Do not add streams after association is closed

Reduce log level to trace for expected case

When remote resets its write half of the stream, we can still send
messages on it. This is not an error condition.

Added TryMarshalUnmarshal for fuzzers

Fuzzers need some public function to perform their work.

Fix SCTP chunk validation

Some chunks were not validated properly what could cause crash.