blpop
BLPOP
BLPOP key [key ...] timeout
BLPOP is a blocking list pop primitive. It is the blocking version of LPOP because it blocks the connection when there are no elements to pop from any of the given lists. An element is popped from the head of the first list that is non-empty, with the given keys being checked in the order that they are given.
Non-blocking behavior
When BLPOP is called, if at least one of the specified keys contains a non-empty list, an element is popped from the head of the list and returned to the caller together with the key it was popped from.
Keys are checked in the order that they are given. Let's say that the key list1 doesn't exist and
