server: Prefer 3 min mix capable outbound peers.

The current logic attempts to maintain at least one mix capable outbound
peer to ensure better connectivity among the mix capable peers.  It
works as intended, however, only having a single mix capable peer means
there will be delays if that peer goes offline until a replacement is
found.  A possible consequence is that any mix clients might potentially
get interrupted if that peer goes offline and a replacement isn't found
quickly enough.

Attempting to maintain a minimum of three mix capable outbound peers
will significantly reduce the likelihood of having periods without any
mix capable peers.

This updates the logic accordingly.

server.go

@@ -1078,24 +1078,34 @@ func (sp *serverPeer) OnVersion(_ *peer.Peer, msg *wire.MsgVersion) {
 		return
 	}
 
-	// Maintain at least one outbound peer capable of supporting p2p mixing.
+	// Maintain a minimum desired number of outbound peers capable of supporting
+	// p2p mixing.
 	if !isInbound && msg.ProtocolVersion < int32(wire.MixVersion) {
-		var hasMixCapableOutbound bool
-		var numOutbound uint32
+		var numOutbound, numMixCapableOutbound uint32
 		peerState := &sp.server.peerState
 		peerState.Lock()
 		peerState.forAllOutboundPeers(func(sp *serverPeer) {
 			if sp.ProtocolVersion() >= wire.MixVersion {
-				hasMixCapableOutbound = true
+				numMixCapableOutbound++
 			}
 			numOutbound++
 		})
 		peerState.Unlock()
 
-		if !hasMixCapableOutbound && numOutbound+1 == sp.server.targetOutbound {
+		const defaultWantMixCapableOutbound uint32 = 3
+		wantMixCapableOutbound := defaultWantMixCapableOutbound
+		if sp.server.targetOutbound < wantMixCapableOutbound {
+			wantMixCapableOutbound = sp.server.targetOutbound
+		}
+		hasMinMixCapableOuts := numMixCapableOutbound >= wantMixCapableOutbound
+		needsMoreMixCapable := !hasMinMixCapableOuts &&
+			numOutbound+wantMixCapableOutbound >= sp.server.targetOutbound
+		if needsMoreMixCapable {
 			srvrLog.Debugf("Rejecting outbound peer %s with protocol version "+
-				"%d in favor of a peer with minimum version %d", sp,
-				msg.ProtocolVersion, wire.MixVersion)
+				"%d in favor of a peer with minimum version %d to maintain "+
+				"min number of mix capable peers (have: %d, target: %d)", sp,
+				msg.ProtocolVersion, wire.MixVersion, numMixCapableOutbound,
+				wantMixCapableOutbound)
 			sp.Disconnect()
 		}
 	}