Updated tor control circuit builder to be able to use multiple circuits
The circuit builder module now has the ability to build and use
different circuit throughput the experiments.  Old format still works,
but now you can pass it arguments like:

time1:node1,node2,node3 time2:node1,node4,node5 time3:node6,node7,node8

Here, the first circuit will be used from time1 to time2, the second
circuit used from time2 until time3, and the last circuit will be used
from time3 until the experiment terminates.

Note: code is not smart enough to automatically order circuits, so make
sure they're put in order (i.e. time1 < time2 < time3).