Add support for MSETEX command

[a-TODO-rov]

Sets the given keys to their respective values.
This command is an extension of the MSETNX that adds expiration and XX options.

Options:

• EX seconds - Set the specified expiration time, in seconds.
• PX milliseconds - Set the specified expiration time, in milliseconds.
• EXAT timestamp-seconds - Set the specified Unix time at which the keys will expire, in seconds.
• PXAT timestamp-milliseconds - Set the specified Unix time at which the keys will expire, in milliseconds.
• KEEPTTL - Retain the time to live associated with the keys.
• XX - Only set the keys and their expiration if all already exist.
• NX - Only set the keys and their expiration if none exist

Reply:
Integer reply: 0 if no keys were set
Integer reply: 1 if all the keys were set