Add support for darner message queue

[bogdanovich]

Darner message queue usues memcache protocol.
https://github.com/wavii/darner

It supports transactional reads, for example:

val, fl, err := memc.Get("task/open")
// Perform job
_, _, _ = memc.Get("task/close")

This PR fixes ReadError that raises because
server response doesn't have "/open" suffix in key name.

Key "task/open" != "task" in server response.

Example:

telnet *** 22133
Escape character is '^]'.
get task/peek
VALUE task 0 24
task_content
END
get task/open
VALUE task 0 24
task_content
END
get task/close
END

[kklis]

Seems fine, thanks!