List references to still open transactions; remove those references once a transaction is committed
Added post-event handler concept, and used it to save the state (if necessary) after a (network) message has been handled
Renamed list() methods to getState(), as preparation to re-use these for state saving
Partially implemented state saving; store all binary data (such as hashes) as hexadecimal
Exclude irrelevant state elements from being saved
Finish state saving, making sure that we always keep an intact state file somewhere, which can be loaded.