buffers: Add very basic producer rate-limiting.
buffers: Add very basic producer rate-limiting.

Only when we run out of space on bridge dirs, so we don't
have to do potentially expensive locking.
This keeps bridge space usage in check when (i) performance
is consumer-limited; (ii) phases are long enough (10M+).
Which tends to cover SPEC and similar simulations.

Change-Id: I0d6e27fa169f3d114609a0fdedf614b08abd4b3d

s-kanev/XIOSim,s-kanev/XIOSim,s-kanev/XIOSim,s-kanev/XIOSim