optimization: reuse last bgsave rdb for full sync

[siddontang]

Now every full sync will do bgsave, which may be slow for large dataset. If we can reuse the last bgsave rdb, and the associated master sync offset is still in the backlog buffer, we may reuse this rdb for full sync and do incremental sync from last master sync offset.

This is only a optimization, and we may do it later, not now.