DISKSTORE TODO
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+* Fix FLUSHALL/FLUSHDB: the queue of pending reads/writes should be handled.
* Check that 00/00 and ff/ff exist at startup, otherwise exit with error.
* Implement sync flush option, where data is written synchronously on disk when a command is executed.
* Implement MULTI/EXEC as transaction abstract API to diskstore.c, with transaction_start, transaction_end, and a journal to recover.
OPTIMIZATIONS
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+* Avoid COW due to incrementing the dict iterators counter.
* SORT: Don't copy the list into a vector when BY argument is constant.
* Write the hash table size of every db in the dump, so that Redis can resize the hash table just one time when loading a big DB.
* Read-only mode for slaves.