cashew: defer all D-Bus signal processing to main loop

Avoid libdbus-c++ deadlock issues with sending dbus messages in dbus
signal callbacks by doing as little as possible in these callbacks.

Instead, have the callbacks schedule dbus signal processing to be done
later from the main loop.

This blanket deferral should be simpler and more robust than trying to
defer specific actions such as Service deletion as we've done in
previous CLs.

BUG=chromium-os:8892
TEST=unit tests, manual testing on device (suspend/resume while
connected to 3G)

Change-Id: I05e9026f2ad253d503d6de702b108a4d261237de

Review URL: http://codereview.chromium.org/5380002