This lets us avoid using Core::deferred_invoke() which is not usable
during application teardown (as there is no event loop to push the
deferred invocation onto.)
(Not that there is an event loop to fire the processing timer during
teardown *either*, but at least we can exit gracefully with pending
timers, unlike deferred invocations, which hang the process. This is an
area where more improvements are definitely needed!)
This patch moves the templated message parsing code to a virtual
try_parse_messages() helper. By doing that, we can move the rest of the
socket draining code up to ConnectionBase and keep it out of line.
This patch splits IPC::Connection into Connection and ConnectionBase.
ConnectionBase moves into Connection.cpp so we don't have to inline it
for every single templated subclass.
