Make ChildEventBusManager.disconnect async.

This allows other async events a chance to be popagated up to
the parent, rather than the bus abruptly being cut off with
a bunch of pending events.

Change-Id: I2a60826174d33240c0a1016b41ef881efdd97fed
Reviewed-on: http://gerrit.dmdirc.com/3818
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager