identityd: gain a bit more insight into the shutdown sequence
identityd: gain a bit more insight into the shutdown sequence

A remote Stop against identityd was observed to hang in production, and
it appears that the ctx.Shutdown never occurred.  The only thing that
happens after <-ShutdownOnSignals is rpcServer.Stop, so adding some
logging around that.

Change-Id: I22198d41c26fb6fb77b4a1e6bb1f612052a0e4c2

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