* Adds ASYNC and SYNC arguments to SCRIPT FLUSH
* Adds SYNC argument to FLUSHDB and FLUSHALL
* Adds new config to control the default behavior of FLUSHDB, FLUSHALL and SCRIPT FLUASH.
the new behavior is as follows:
* FLUSH[ALL|DB],SCRIPT FLUSH: Determine sync or async according to the
value of lazyfree-lazy-user-flush.
* FLUSH[ALL|DB],SCRIPT FLUSH ASYNC: Always flushes the database in an async manner.
* FLUSH[ALL|DB],SCRIPT FLUSH SYNC: Always flushes the database in a sync manner.
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