#12: use flush instead of flushAsyn when writing EOF
Enhance error handling: split exception from the Error frame (see also #10)
dartdoc
minor
Use only subscriptionId for subscription matching

Based on STOMP 1.2 spec, destination of SUBSCRIBE sent frames
and MESSAGE received frame SHOULD be the same, that means they
also could be different. As a consequence, only the subscriptionId
should be used for subscription matching.