Spawn multiple assimilation requests if a low threshold is met

[nomel7]

If we are really below minPeers() then TrPeerManager should try spawn multiple assimilation requests in order for the node to try get back up to speed.

[sanity]

TrPeerManager. maintenance() will spawn new assimilation attempts if peers.size() < minPeers, is something additional required?

[sanity]

ok, i see, the suggestion is we should do several. Let's see how well the current code works in practice first.

[sanity]

So non-blocker for alpha release