lib/config, lib/connections: Add optional connection limits (fixes #7176) (#7223)

This adds two new configuration options:

    // The number of connections at which we stop trying to connect to more
    // devices, zero meaning no limit. Does not affect incoming connections.
    ConnectionLimitEnough int

    // The maximum number of connections which we will allow in total, zero
    // meaning no limit. Affects incoming connections and prevents
    // attempting outgoing connections.
    ConnectionLimitMax int

These can be used to limit the number of concurrent connections in
various ways.

lib/model/model_test.go

@@ -3407,7 +3407,7 @@ func TestDeviceWasSeen(t *testing.T) {
 	if err != nil {
 		t.Error("Unexpected error:", err)
 	}
-	entry := stats[device1.String()]
+	entry := stats[device1]
 	if time.Since(entry.LastSeen) > time.Second {
 		t.Error("device should have been seen now")
 	}