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/api/mocked_model_test.go

@@ -49,11 +49,15 @@ func (m *mockedModel) FolderProgressBytesCompleted(_ string) int64 {
 	return 0
 }
 
+func (m *mockedModel) NumConnections() int {
+	return 0
+}
+
 func (m *mockedModel) ConnectionStats() map[string]interface{} {
 	return nil
 }
 
-func (m *mockedModel) DeviceStatistics() (map[string]stats.DeviceStatistics, error) {
+func (m *mockedModel) DeviceStatistics() (map[protocol.DeviceID]stats.DeviceStatistics, error) {
 	return nil, nil
 }