service: Allow multiple clients on the same connection

This is needed to ease the implementation of the Flatpak location portal.

Fixes #79.
3 jobs for wip/multi-client in 7 minutes and 9 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #37966
ubuntu-16.04

00:07:09

passed #37967
ubuntu-17.10

00:06:44

passed #37968
ubuntu-18.04

00:06:35