Add health check endpoint

[orangejulius]

It would be great if all the Pelias services had a health check endpoint. Currently Placeholder does not.

The API uses /status, so it would be great if Placeholder did as well for consistency.

[missinglink]

The /parser/search & /parser/findbyid endpoints would be good candidates for this.

What is the intent of the /status endpoint, should it be a 'dumb' endpoint which doesn't hit the database or a 'smart' one which does?