Add an endpoint to query the transfer journal

[jeff-cohere]

Now that we've got an embedded transfer journal in place for the DTS, we need an endpoint to query for transfer metadata over a time interval. The request need only include the temporal endpoints for the query interval [start, stop). The response would be a JSON array of journal.Record structs, which should be straightforward to construct from the data structures in the journal.

[jeff-cohere]

This was done in #173, but still needs testing.