Support GET option for SET command

[talbii]

It is possible to supply the GET argument when setting a new string value, in order to return the old value (this is equivalent to GETSET). Currently Dragonfly supports this (i.e., there is no syntax error), however an "OK" is returned instead of the old value.

Example

> SET k1 "Hello"
"OK"
> SET k1 "World" GET
"OK" # expected: "Hello"
> GET k1
"World"

[romange]

#484 is related to this.