Add more tests

[mostafa]

Target is at least 80% coverage. The coverage report (on coveralls.io) shows which parts needed test the most. It's mostly about functionality and unhappy path testing (in error cases).

• gRPC server and HTTP gateway
• Plugin Registry (load plugin, register hooks, etc.): needs more work, because there needs to be a plugin to load and register its hooks.
• Network proxy: needs more work in other milestones, especially after Enable multi-pool client connections #398.

Depends

• Graceful shutdown #439