allow for libmunge calls to be made asynchronously

[dun]

Allow for libmunge calls to be made asynchronously. When these calls are made in an event loop, the process making the call is blocked while waiting for a response and can't process new work in the interim.

https://funcptr.net/2013/04/20/embedding-zeromq-in-the-libev-event-loop/

Requested by Flux: since there is no asynchronous use of libmunge, job-ingest blocks while in libmunge; need some way to use the interface asynchronously. This mostly affects the system instance (i.e., multi-user jobs).