Allow process poll rate to be customized

Hi Nick, I've been digging into Stem test performance with an aim of making them faster. Our tests regarding OwningControllerProcess take fifteen seconds each because that's the poll rate of the tor process. Looks like this is defined at...

https://gitweb.torproject.org/tor.git/tree/src/common/procmon.c#n165

Would you mind making this customizable (probably via a hidden double-underscore torrc options)? This will let me make our tests run faster for us both. :)

Thanks!