Break into a python interpreter from anywhere

Summary:
I use pdb a lot when debugging and I find it really annoying that we only get stack traces from errors on gunks. Wouldn't it be cool to be able to at least have access to an interpreter at the bottom of a stack trace? Well, rdb does this. To use it, insert a call to "rdb.break_to_interpreter" and the greenlet will block while waiting for a telnet (or netcat if you can) connection on localhost (and without blocking the whole process).

Currently the debugger is very basic - it's still possible to examine the stackframe using sys._getframe, though. I'm looking for a way to make the frame data available in locals() and to have greenlets break into the debugger whenever they get an uncaught exception.

Test Plan: Ran a sync

Reviewers: charles, spang, emfree

Reviewed By: emfree

Projects: #inbox-sync

Differential Revision: https://review.inboxapp.com/D321