Merge lp://qastaging/~afrantzis/mir/glib-main-loop-spike-wip into lp://qastaging/mir

Please don't spend time code reviewing this branch, it's only meant to
communicate the general approach taken for the GLib version of the main
loop. However, please do take a look and let me know if you feel this is
less/more clear/straightforward than our asio version and whether you
like the general direction or not. See [1] for the way glib sources
work, which should aid your understanding.

Some integration tests are failing/hanging with this branch and the
culprit is the signalfd based signal source. The problem turns out to be
that in order for signalfd to catch a signal, the signal needs to be
blocked in *all* threads. Usually that translates to blocking the
signals in main() so all threads will inherit the signal mask.
Unfortunately, this doesn't work for us since we are a library and can't
enforce a signal mask in the general case.

Next steps:

1. Replace signalfd with something else (probably just signal handlers
   + self-pipe to notify main loop), or (less likely) find a way to
   enforce a global signal mask.

2. Start proposing the changes in cleaned-up, small steps:
   * Infrastructure changes (e.g. ThreadSafeList, Clock)
   * GLibMainLoop basic start/stop + tests
   * GLibMainLoop signal handing + tests
   * GLibMainLoop fd handing + tests
   * GLibMainLoop timers + tests
   * GLibMainLoop queued events (ServerAction) + tests
   * GLibMainLoop as a proper MainLoop extension, and
     a mechanism to select it instead of AsioMainLoop

[1] https://developer.gnome.org/glib/stable/glib-The-Main-Event-Loop.html#GSourceFuncs