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/connections/structs.go

@@ -18,6 +18,7 @@ import (
 	"github.com/syncthing/syncthing/lib/config"
 	"github.com/syncthing/syncthing/lib/nat"
 	"github.com/syncthing/syncthing/lib/protocol"
+	"github.com/syncthing/syncthing/lib/stats"
 
 	"github.com/thejerf/suture/v4"
 )
@@ -193,9 +194,11 @@ type genericListener interface {
 type Model interface {
 	protocol.Model
 	AddConnection(conn protocol.Connection, hello protocol.Hello)
+	NumConnections() int
 	Connection(remoteID protocol.DeviceID) (protocol.Connection, bool)
 	OnHello(protocol.DeviceID, net.Addr, protocol.Hello) error
 	GetHello(protocol.DeviceID) protocol.HelloIntf
+	DeviceStatistics() (map[protocol.DeviceID]stats.DeviceStatistics, error)
 }
 
 type onAddressesChangedNotifier struct {