Atomically sets key to value and returns the old value stored at key.
Returns an error when key exists but does not hold a string value.
Design pattern
GETSET can be used together with INCR for counting with atomic reset.
For example: a process may call INCR against the key mycounter every time
some event occurs, but from time to time we need to get the value of the counter
and reset it to zero atomically.
This can be done using GETSET mycounter "0":
INCR mycounter
GETSET mycounter "0"
GET mycounter@return
@bulk-string-reply: the old value stored at key, or nil when key did not exist.
@examples
SET mykey "Hello"
GETSET mykey "World"
GET mykey