allow for multiple requests in a single connection to munged

[dun]

MUNGE clients perform a single request per connection to the daemon. This results in an increased request latency if a client needs to perform many such requests (e.g., bulk submit), since each request requires a new connection to be established and subsequently torn down. Instead, allow clients to perform multiple requests over a single connection to the daemon.

Requested by Flux: "when built --with-flux-security, our ingest rate drops because each job requires RTT to munge daemon to sign, plus connection setup and teardown".