sim: tested concurrency over actions

sim: tested concurrency over actions

This demonstrates concurrent behavior that looks right. And with a
random seed, the results are deterministic.

init foo
init bar
init backup
connect foo <-> bar
connect foo <-> backup
addmulti 10 testfiles 1mb 1gb foo backup
action foo gitpull backup
wanted foo nothing
wanted bar anything
wanted backup anything
action bar gitpull foo
action foo dropunwanted while action bar getwanted foo