feat(server): add support for GET options with SET command #389

[Super-long]

This PR adds the get option to stringfamily.

Two modifications.

1. add memcache_reply_builder/redis_reply_builder, replace the existing static_cast.
2. StringFamily::Set returns a different return value to the client in the presence of SetCmd::SET_GET.

A test.

1. perform a simple functional test on set key value get.