Returns the string representation of the type of the value stored at key.
The different types that can be returned are: string, list, set, zset
and hash.
@return
@simple-string-reply: type of key, or none when key does not exist.
@examples
SET key1 "value"
LPUSH key2 "value"
SADD key3 "value"
TYPE key1
TYPE key2
TYPE key3