Network max connections and peer management

[ferranbt]

Configuration options on the server for the max number of connections and the max number of incoming connections allowed. Dialing tasks are idle if there are no more slots to open a connection.

Syncer (or any other service connected to the network) should be able to set a peer as unnecessary at any time (i.e. server.SetUnnecessary(peerID string)). Server would evict that peer from the peerset and dial for another one.