Investigate using a socket rather than a TCP port

[GeertJohan]

Avoids conflicts with existing programs using the tcp port, and avoids accidents where an external program connects to 'our' postgres instance.

[provokateurin]

https://github.com/fergusstrange/embedded-postgres doesn't support sockets, but fergusstrange/embedded-postgres#128 would be an alternative solution (that we could also build into trek itself).