feat(config): enable multiple connections by default (#10151)

This changes the default number of connections from one to three (one
metadata + two data connections). This should give some advantages of
multiple connections, while also not being an overwhelming change for
larger installations. (Though those may need to tweak their settings
anyway, as always.)

lib/config/deviceconfiguration.go

@@ -13,7 +13,7 @@ import (
 	"github.com/syncthing/syncthing/lib/protocol"
 )
 
-const defaultNumConnections = 1 // number of connections to use by default; may change in the future.
+const defaultNumConnections = 3 // number of connections to use by default
 
 type DeviceConfiguration struct {
 	DeviceID                 protocol.DeviceID `json:"deviceID" xml:"id,attr" nodefault:"true"`