Support receiving fragmented STOMP frames

Support receiving STOMP frame split into multiple Websocket
messages, since this is the current behavior of Stomp.js and
Dart STOMP library when frames are bigger than 16 Kbytes.

A new maxFrameBufferSize parameter (default 64 Kbytes) has
been added in order to allow to configure the maximum
size of the buffer used to assemble each fragmented STOMP
frame.

This commit also introduces FragmentedMessageAssembler and
AbstractFragmentedMessageAssembler in order to provide
generic helpers to create a message from multiple fragments.

Issue: SPR-11527