[management] Add Redis-backed kill switch for Sync fast path

Gate the peer-sync fast path on a runtime flag polled from Redis so operators can roll the optimisation out gradually and flip it off without a redeploy.

Without NB_PEER_SYNC_REDIS_ADDRESS the routine stays disabled, every Sync runs the full network map path, and no entries accumulate in the peer serial cache — bit-for-bit identical to the pre-fast-path behaviour. When the env var is set, a background goroutine polls the configured key (default "peerSyncFastPath") every minute; values "1" or "true" enable the fast path, anything else disables it.

- RunFastPathFlagRoutine mirrors shared/logleveloverrider: dedicated Redis connection, background ticker, redis.Nil treated as disabled.
- NewServer takes the flag handle; tryFastPathSync and the recordPeerSyncEntry helpers short-circuit when Enabled() is false.
- invalidatePeerSyncEntry still runs on Login regardless of flag state.
- NewFastPathFlag(bool) exposed for tests and callers that need to force a state without going through Redis.

client/cmd/testutil_test.go

@@ -135,7 +135,7 @@ func startManagement(t *testing.T, config *config.Config, testFile string) (*grp
 	if err != nil {
 		t.Fatal(err)
 	}
-	mgmtServer, err := nbgrpc.NewServer(config, accountManager, settingsMockManager, jobManager, secretsManager, nil, nil, &mgmt.MockIntegratedValidator{}, networkMapController, nil, nil)
+	mgmtServer, err := nbgrpc.NewServer(config, accountManager, settingsMockManager, jobManager, secretsManager, nil, nil, &mgmt.MockIntegratedValidator{}, networkMapController, nil, nil, nil)
 	if err != nil {
 		t.Fatal(err)
 	}