conncache: Fix cancellation leaks upon close.
conncache: Fix cancellation leaks upon close.

Change-Id: I3f951cab94423fb43fd36d78112f50367e97dc54

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref