Spawn a remote debugger in a separate greenlet

Summary: This code adds support for spawning a remote debugger if the DEBUG_CONSOLE_ON is specified. The debuggers listens on localhost on port $PID for telnet connections. It gives access to a simple REPL with the possibility of using the heapy heap profiler to inspect the heap.

Test Plan: Ran the code.

Reviewers: emfree, charles

Reviewed By: charles

Projects: #sync-engine

Differential Revision: https://review.inboxapp.com/D637