audit all __repr__ methods

All of the repr methods should be audited and updated to be actually useful and helpful, if/when possible.

My notes below:

- audit all __repr__ methods.
        - EventListener (won't be able to eval b.c. of the filter func)
                - name(s)
                - filter-func repr?...
                - add length of history
                - add length of pending queue (is this possible?)
        - Events
                - number of registered listeners?
        - Message       [DONE]
        - AsyncProtocol [DONE]
        - Models        [DONE]
        - QMPClient
                - number of registered listeners
                - number of pending exec statements
                - await greeting y/n
                - negotiate y/n