Add unix domain socket support

[Beaness]

Deprecates ServerInfo#getAddress and errors if it is a unix domain socket, current velocity proxies setups should remain working.
Adds a new method ServerInfo#getSocketAddress as alternative.

Based on #991, #422, b003890, b003890

[WouterGritter]

I wonder what your usecase for this feature is? I think the 'modern' go-to solution for a sort of 'anonymous'/intermediate proxy not exposed to the world would be to use Docker and Docker's networking.

[SirYwell]

I wonder what your usecase for this feature is? I think the 'modern' go-to solution for a sort of 'anonymous'/intermediate proxy not exposed to the world would be to use Docker and Docker's networking.

Using Docker still allows using Unix Domain Sockets, and the performance will be (slightly) better most likely.

[Beaness]

I wonder what your usecase for this feature is? I think the 'modern' go-to solution for a sort of 'anonymous'/intermediate proxy not exposed to the world would be to use Docker and Docker's networking.

I think most people would use this for better performance when having multiple servers on the same machine. As you said there are different (or even better) alternatives for networking security.