swarm/simulation: wait till snapshot connections are recreated

[zelig]

#1284 introduces WaitTillSnapshotRecreated
this function used in simulations is blocking until all the connections specified
in the snapshot are actually up and running and show in the both node's kademlias.
It differs from WaitTillHealthy, which waits only until all the kademlias are
healthy (it might happen even before all the connections are established).

I think this functionality should be part of snapshot loading at least when using swarm/network/simulation, then all WaitTillHealthy calls could be eliminated from snapshot-based tests