Add support for `FLUSHDB` and `FLUSHALL` commands

[ashwaniYDV]

FLUSHDB

• Syntax FLUSHDB [ASYNC | SYNC]

• Delete all the keys of the currently selected DB.

• By default, FLUSHDB will synchronously flush all keys from the database.

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

  • ASYNC: flushes the database asynchronously
  • SYNC: flushes the database synchronously

FLUSHALL

• Syntax FLUSHALL [ASYNC | SYNC]

• 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 keys from the database.

• 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