Set `key` to hold the string `value` and set `key` to timeout after a given
number of seconds. This command is equivalent to executing the following
commands:

```
SET mykey value
EXPIRE mykey seconds
```

`SETEX` is atomic, and can be reproduced by using the previous two commands
inside an `MULTI` / `EXEC` block. It is provided as a faster alternative to the
given sequence of operations, because this operation is very common when Redis
is used as a cache.

An error is returned when `seconds` is invalid.

@return

@simple-string-reply

@examples

```cli
SETEX mykey 10 "Hello"
TTL mykey
GET mykey
```