Remove the existing timeout on key, turning the key from volatile (a key
with an expire set) to persistent (a key that will never expire as no timeout
is associated).
Examples
127.0.0.1:6379> SET mykey "Hello"
OK
127.0.0.1:6379> EXPIRE mykey 10
(integer) 1
127.0.0.1:6379> TTL mykey
(integer) 10
127.0.0.1:6379> PERSIST mykey
(integer) 1
127.0.0.1:6379> TTL mykey
(integer) -1