This commit introduces the consensus parameter "circ_max_cell_queue_size"
which controls the maximum number of cells a circuit queue should have.

The default value is currently 50000 cells which is above what should be
expected but keeps us a margin of error for padding cells.

Related to this is #9072. Back in 0.2.4.14-alpha, we've removed that limit due
to a Guard discovery attack. Ticket #25226 details why we are putting back the
limit due to the memory pressure issue on relays.

Fixes #25226



Signed-off-by: David Goulet <dgoulet@torproject.org>