Extend databroker-unpack to load documents into Mongo

[danielballan]

For many use cases, running intake top of a directory of mspgack or jsonl files is not suitable. The databroker-unpack utility should provide an option to copy Documents from the files into MongoDB and auto-generate a suitable catalog file.

I didn't do this in the first pass because it was not obvious to me how to spell, or how we might want to extend it. Initial thought:

databroker-unpack inplace DIRECTORY NEW_CATALOG_NAME  # current functionality

and

databroker-unpack mongo_normalized DIRECTORY NEW_CATALOG_NAME --metadatastore=MONGO_URI --filestore=MONGO_URI

Now, databroker-pack is intentionally "opinionated" and only supports a hard-coded subset of suitcases (msgpack, jsonl). Should we do the same for -unpack? I think so, because it enables us to auto-generate a working catalog file.

[tacaswell]

Maybe a --merge flag that will try to insert into an existing catalog (rather than creating a new one). You can imagine having behind the scenes logic to sort out if we can side-load (ex msgpack -> msgpack) by just copy files, but fallback to db_new.insert(...).

We probably need a set of helpers for creating new catalogs (of what ever backing) and dumping the config files in the correct place.

[danielballan]

Both done in #16