Instead of caching pending messages during the handshake and relaying
them afterwards, they are now sent after the handshake has been
completed.
This is possible because the relevant processors (messages and status
changes) are now event handlers which are registered at the event bus,
which means that they can immediately react to pending messages even if
Client#initReceiver has not been fully executed yet.
Because Client#initReceiver exists for that very reason, it is now
longer necessary. ID generator initialization, which was its other part,
is now directly handled in Startup#performHandshake, which is a far more
sensible placement.
Fixes#106
