Provide healthcheck in service repo

[karlb]

There are basic healthchecks in the raiden service bundle repo. Having these checks in this repo would be preferable, since that would allow the service to test and update the checks as needed.

See https://github.com/raiden-network/raiden-service-bundle/blob/master/docker-compose.yml#L73

Potential other improvements:

• Use docker healthcheck feature
• Provide healthcheck for PFS (accessing the info endpoint should be good enough)
• Provide healthcheck for RC
• Clean up healthcheck for MS

[palango]

Related to #650