New API surface, smaller changes and fixes

* Retransmission listeners introduced for easier UX
* Protocol API send & receive methods rework to handle
byte arrays and ByteBuffers instead of user-data directly
** Avoids fragmentation in internet layer
** The ProtocolHost still handles conversion of user
objects
** Byte data is split into multiple packets < MTU
for transmission by user application
* ProtocolHost now accumulates multiple to-be-sent
user-datas < MTU into one single packet for transmission
efficiency
* ProtocolHost is no longer tied to a single datatype
* Fix Synchronization example to relay ordered data
properly
* Various other smaller changes and fixes

Change groupId and package names to com.github.mucaho.

Remove unnecessary javadoc tag. [skip ci]