lib/model, lib/weakhash: Hash using adler32, add heuristic in puller

Adler32 is much faster, and the heuristic avoid the obvious cases where it
will not help.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3872

lib/config/config_test.go

@@ -108,6 +108,7 @@ func TestDeviceConfig(t *testing.T) {
 				Versioning: VersioningConfiguration{
 					Params: map[string]string{},
 				},
+				WeakHashThresholdPct: 25,
 			},
 		}