feat: graceful shutdown

[meling]

We currently don't do any graceful shutdown for our quorum calls and other call types. We should support graceful shutdown.

[meling]

When using multicast or unicast, the client receives no error signaling a graceful shutdown, i.e., to stop sending requests. That is, the Gorums runtime should recognize this error (graceful shutdown) and handle it appropriately, even for multicast/unicast.