Detect if we try to put a cell onto a supposedly blocked cell queue.

Detect if we try to put a cell onto a supposedly blocked cell queue.

When this happens, run through the streams on the circuit and make
sure they're all blocked.  If some aren't, that's a bug: block them
all and log it!  If they all are, where did the cell come from?  Log
it!

(I suspect that this actually happens pretty frequently, so I'm making
these log messages appear at INFO.)