Inserts value at the tail of the list stored at key, only if key already
exists and holds a list.
In contrary to RPUSH, no operation will be performed when key does not yet
exist.
@return
@integer-reply: the length of the list after the push operation.
@examples
RPUSH mylist "Hello"
RPUSHX mylist "World"
RPUSHX myotherlist "World"
LRANGE mylist 0 -1
LRANGE myotherlist 0 -1