metadata generation improvements

[melange396]

• metadata generation routine can get the list of src/sig faster now by using just the signal dimension TABLE and not the big complicated VIEW to gather them
• add sort_keys=True to json.dumps() to ensure deterministic output
• add number of points to metadata for each aggregation since we are in there already... and maybe avg number of re-issues to metadata if we wanna also hit the full table

https://github.com/cmu-delphi/delphi-epidata/blob/dev/src/acquisition/covidcast/database.py#L494

[melange396]

this should also include adding a "minimum issue" as described in #232

[melange396]

Another idea:

Use the active attribute of signals from delphi.epidata.server.endpoints.covidcast_utils.model.data_sources to limit metadata generation to only the signals that are currently being updated (similar to how the one metadata endpoint filters "active" signals). The downside is that this presumes the signals spreadsheet is updated and accurate, and is in sync with the local CSV version.

• Update COVIDcast metadata on insert #289 still allows for a more efficient limiting/filtering of metadata generation, but the approach described here would be an easier improvement to implement.