neutrino: standardize use of connmgr.Remove and connmgr.Disconnect
neutrino: standardize use of connmgr.Remove and connmgr.Disconnect

This commit ensures proper management of the connection manager's
internal state. The connnection manager's Remove and Disconnect will now
only be handled in two places, within handleDonePeerMsg and
outboundPeerConnected. Anytime we wish to disconnect a peer, we only
need to invoke its Disconnect method, which will trigger a call to
handleDonePeerMsg where we determine whether we should simply disconnect
the peer and retry if they are persistent, or disconnect completely and
try a new peer.

lightninglabs/neutrino