Remove Pykka.

Pykka's ThreadingFuture object is interacting with ArchiveBot's wpull
hooks and wpull in weird ways; see
https://github.com/chfoo/wpull/issues/186 for an example.  Additionally,
I haven't been able to use Pykka for the purpose I wanted to use it for
(async Redis send/receive), so it seems like the best course of action
is to remove it.
Synchronize access to settings data.

The settings listener runs in one thread; wpull runs in another.