The main() function is growing too big (142 lines as of the date of this
commit), so this attempts to extract some functionality out of there and
into their own functions to make it easier to reason about them and keep
functions short and concise.
Every time a JSON object is returned in an HTTP response, the
appropriate header needs to be set and the object itself needs to be
encoded. Doing this in every function is repetitive and error prone
(getDBFile and postDBScan, for instance, never set any headers).
This adds a helper function to centralize the appropriate JSON response
handling.