Delete all the keys of all the existing databases, not just the currently
selected one. This command never fails.

By default, `FLUSHALL` will synchronously flush all the databases. Starting with
Redis 6.2, setting the **lazyfree-lazy-user-flush** configuration directive to
"yes" changes the default flush mode to asynchronous.

It is possible to use one of the following modifiers to dictate the flushing
mode explicitly:

- `ASYNC`: flushes the databases asynchronously
- `!SYNC`: flushes the databases synchronously

Note: an asynchronous `FLUSHALL` command only deletes keys that were present at
the time the command was invoked. Keys created during an asynchronous flush will
be unaffected.

@return

@simple-string-reply

@history

- `>= 4.0.0`: Added the `ASYNC` flushing mode modifier.
- `>= 6.2.0`: Added the `!SYNC` flushing mode modifier and the
  **lazyfree-lazy-user-flush** configuration directive.