Support ASYNC/SYNC flush modes for flushDb() and flushAll()

See https://redis.io/commands/flushdb and https://redis.io/commands/flushall

FLUSHDB:
- Both modes are supported since Redis v1.0.0

FLUSHALL:
- ASYNC since v4.0.0
- SYNC since v6.2.0

On a cursory glance, it also appears that both Lettuce and Jedis support these modifiers already.