mirage/mirage-server: revise connection management

[hannesm]

it's pretty ad-hoc atm, what is in general (server/resolver have slightly different use cases) there:

• keep one tcp connection to remote
• keep this interesting tcp connection of remote
• only establish a single tcp connection to each remote (esp. when primary boots up with 10 zones, and has a secondary for all zones -- "block" until tcp session established)
• only keep n tcp connections around at any time

[avsm]

This looks like it's generally a useful bit of logic to have in a tcp client somewhere; irmin might need it too for a p2p backend /cc @samoht

[hannesm]

addressed in #184.