feat: Allow shutdown of Provider (#121)

This adds Provider::shutdown allowing to stop the Provider without
having to abort it.  On the wire however it still does not do anything
graceful: all the connections are stopped and any in-transit data is
lost.

Additionally this adds tracing spans to the connection handling.

Finally it fixes a bug where read_lp would not return None if the
connection was closed normally.

Part of #77 but this is not very graceful on the wire yet.