This is a breaking change - see BREAKING-CHANGES.txt

Following commit 093dbc7df1 pings of a
child process connection need to initialised explicitly. This commit
adds a missing initialisation.

Also allows optionally ignoring callbacks during disconnect, so that the
call to `connectionLost` can be bypassed when disconnect is called from
the derived class destructor.

It was possible to encounter data races when when requesting connection
callbacks on the message thread, but creating/destroying connection
objects on a background thread.

This change ensures that a message will not be processed if the
destination connection is destroyed before the message is delivered.