redis: `unixSocket` broken for relative socket paths

[shivaraj-bh]

I think this doesn't work as intended, if I specify unixSocket = "redis.socket";, which is also a relative path, the socket will be created at data/r1/redis.socket. I think the check should be ! lib.hasPrefix "/" config.unixSocket.

Originally posted by @niklasmohrin in #353 (comment)