Switch TCP sockets to UNIX sockets

The AKTIN update agent uses TCP sockets for data warehouse communication. For the new communication style planned, @akomii requested switching from TCP sockets to UNIX sockets. We tried once but failed, because we needed a solution to communicate with docker containers.

## Difference between TCP- and UNIX sockets
| attribute | TCP | UNIX |
| -- | -- | -- |
| network interface | tcp | internal gateways |
| addressing | port | file |
| discovery | network-wide | local only |
| cleanup | not necessary | manual file system cleanup |
| compatibility | broad | unix only |