CFG: Prevent CFG orignating messages during SYNC

During SYNC, corosync-cfgtool -R/-H commands can pass through IPC then
send totem messages. This may corrupts
assembly_list_inuse/assembly_list_free if those messages are recedived
after SYNC is done.

The solution is marking related CFG APIs as
CS_LIB_FLOW_CONTROL_REQUIRED.

Signed-off-by: Jason HU <huzhijiang@gmail.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>