Supports sockets for praefect
Right now praefect doesn't support sockets but just TCP. Sockets are used a lot at GitLab, and might make the GDK integration better.
The main use case is actually where the GitLab-rails and praefect would be on the same machine but the Gitaly nodes storing are on different. The communication between the clients on the same machine could be done through sockets. This has advantages, namely speed; sockets are a bit faster than TCP. Secondly; security wise its nice to be able to place a socket in a directory where the access to directory can be guarded with chmod.