Consider using asyncio

[mystor]

The vast majority of phlay's runtime is spent doing network I/O or calling expensive subprocesses. If we switch to using asyncio instead, we can start running these processes in parallel, which might make it run faster.

The primary motivator for this is git-cinnabar, which takes a while to perform the git->hg conversions required to get the mercurial sha1s. As this is a fairly reliable process, and shouldn't affect whether or not the user would want to push their commits, it would be nice to start it early in the process, but not block on its completion until after the user types 'y'.

There are also a series of conduit queries which are somewhat slow and might be nice to parallelize.

Fortunately, as we depend on python >= 3.6, we have access to asyncio, which should make this much easier.