implement reusable proxy connections

[garlick]

[snip] I was thinking more along the lines of how ssh control sockets work. When you ssh to host, if control sockets are enabled ssh first looks for a socket in a well known location based on the the target host and options (like username, port, etc). Could the ssh connector do something similar to re-use an existing proxy connector before launching another ssh? If one doesn't exist, flux-proxy could be started running daemonized.

Originally posted by @grondo in #3999 (comment)