Implement a placeholder mechanism in the channel,id->circ map

We'll use this to help fix bug 7912, by providing a way to mark
that a circuit ID can't get reused while a DESTROY is queued but not sent.
Code to track on a circuit whether it has a "pending" delete cell

This will be used in a fix for bug7912.
Implementation of a fix for bug 7912

I added the code to pass a destroy cell to a queueing function rather
than writing it immediately, and the code to remember that we
shouldn't reuse the circuit id until the destroy is actually sent, and
the code to release the circuit id once the destroy has been sent...
and then I finished by hooking destroy_cell_queue into the rest of
Tor.
Add destroy balance tracking and logging to circuitmux
Add a changes file for bug 7912

I'm calling it a bugfix on 0.2.0.1-alpha, since that's where cell
queues were first introduced.
Fix compile warnings wrt printf formating of int64_t
Merge remote-tracking branch 'public/bug7912_squashed'