Sets the given keys to their respective values.
MSET replaces existing values with new values, just as regular
MSETNX if you don't want to overwrite existing values.
MSET is atomic, so all given keys are set at once.
It is not possible for clients to see that some of the keys were updated while
others are unchanged.
MSET can't fail.
MSET key1 "Hello" key2 "World" GET key1 GET key2