Do not disconnect from synced peers

[kyle-u410]

🚀 Feature

When snarkOS does a periodic refresh of peers, only select peers that are in sync when there are more than one.

Motivation

When only some peers are caught up with chain, it could be that only one or two peers can help keep snarkOS in sync. When these peers are removed due to periodic refresh, then the current node becomes out of sync. The periodic cycling is good, but the loss of only peer that is in sync is bad. So only remove an in sync peer when there are a number of them in the peer list.