Research ways to re-use existing instances of PTs

Currently fog launches a new child process for each PT that is needed.

It would be more efficient to re-use a PT that was already launched by Tor (e.g. to use for a normal non-chained singleton PT), but all the ways of doing this I can think of are quite messy.

So we may never actually want to implement this ticket, but I am making it to record the issue.