/volume API endpoint(s) need to determine the meter "type" and perform the correct calculation

Each meter event includes information about whether the meter is a delta or absolute value. The purpose of that field in the meter was to let the API figure out the right way to respond to a question about the volume for a meter in a query. We are adding explicit query endpoints for /duration, /volume/max, and /volume/sum, but have left the automatic detection logic for the /volume endpoint for later.

To implement this we will need to add a method to the DB API to get metadata for a meter by name, which means tracking that required metadata somehow. That will also make it easier to implement the REST API endpoints for querying the known meters.