Convert `get_protocol_state` to an automatic property, so that rather
than requiring explcit signals for lifecycle events, a GObject notify
signal can be used instead.
Convert disconnect signal to a property so it can be accessed if needed.
Convert code listening to the disconnect signal to listen to notify
signals for `protocol_state` instead, and hence also treat the session
as disconnected when a logout is in progress.
Fixes#986
Convert mailbox and mailbox RW/RO state accessors to properties and
rename to be more explicit about what they mean. Remove mailbox arg
from ::getProtocolState() since it's unused.
Allow specifying the connect greeting timeout length, ensure that
any connect errors are in place before releasing the connect waiter,
add unit test to ensure it works properly.
