[RFC] SAVE/BGSAVE: add IPC file-lock to serialize SAVE/BGSAVE across instances

In order to make redis scale on modern SMP machines, we usually spawn multiple instances at the same time. However, when more than one of the SAVE/BGSAVE processes interleaving with each other, disk IO will be throttled. So we add this feature to serialize this io intensive routine to reduce IO contention in our system.