Use an event loop to coordinate async SDK work

[aabmass]

Originally from #62 (comment)

For example handling periodic exporting in BatchSpanProcessor and PeriodicExportingMetricReader could be done in a single asyncio event loop (using a single thread) across all instances of all of these objects.

We could avoid creating a new background thread in each instance of those classes and it would simplify the synchronization logic. Would work nicely with async exporters and async callbacks which could all run in the same event loop #3273.