discrete_log initialization doesn't work if there isn't an "event loop"

[danwallach]

I'm working on getting EG to work with ray.io, a cluster computing framework. I ran into one issue with async code in dlog.py. The gist of it is that the call to initialize __dlog_lock was failing, and I was getting an error about a missing "event loop". Turns out, the way Ray was running the code was tickling this issue.

The solution is really straightforward: delay initialization of the lock variable until you're inside __discrete_log_internal.

[keithrfung]

No need to mention a PR is incoming. Simply assign yourself to the issue.