Small Async API fixes

[DemetrisChr]

• Add missing close_connection and reset_client in core _ClientAdapter.
• Don't ignore the event loop parameter in AsyncCluster.create_instance()
• Remove Event loop is not running check when creating an AsyncCluster (?)
• Update C++ core

[thejcfactor]

I think we can either:

1. move the running event loop check into the loop property because that would be the point at which we actually need the event loop
2. Rely on the Python environment to complain about a non-running event loop.

With the current changes, we have #2 already so lets see if that does all that we need.