Allow easily running event processing "*not* in a background thread"

[goodboy]

By default switchio's internal event loop spawns a background thread which runs an asyncio event loop for processing events from FS. The original reason for this was legacy (before porting to asyncio) but still serves a useful purpose - that you can run switchio non-blocking alongside other blocking frameworks or programs from a single python process.

Now that this project is officially python 3.5+ only and has full support for asyncio we should allow users to be able to run event processing all in thread (using one asyncio loop) if desired.

The changes required for this shouldn't be too hard, but we'll need to mess with some plumbing:

• don't launch a background thread on ESL connect
• make it possible to pass in an asyncio loop (that can be run from the top most level) all the way down to core machinery
  • a Client needs to support a loop arg to allow passing it down to the contained listener and any other connections
  • the EventListener kind of already supports it since you can pass in an asyncio loop when creating a new EventLoop (see below)
  • the internal EventLoop already supports this

See @enp's example app using just the switchio.api.Connection in #61 for the original use case.