Quick milestone fixes

[jisantuc]

Overview

This PR adds a few quick fixes to support the first tagged release. It:

• Adds request logging in a nice json format that plays nicely with cloud service logs parsers
• Adds a cascade to the item reference on histograms so items that you put in a histogram can ever be deleted
• Returns a 404 instead of an empty tile with a 200 when you make a request for a mosaic that's not in a collection
• Includes the limit value in the search context property

Checklist

• New tests have been added or existing tests have been modified These things aren't really testable aside from "didn't break the server", which is exercised in existing tests, mostly, except for mosaics, which have their own "figure out how to test this better" issue (Properly test mosaic routes #829)

Testing Instructions

• just CI

Closes #255

Closes #884

Closes #859

Closes #793