Add tile rendering for features

[hampelm]

Ready for review.

I still want to add a tile zoom limit (specifically return a blank tile at far zooms)

[hampelm]

Will transfer / respond to questions from #144 later today so we can keep track of them

[hampelm]

From #147:

It'd ideally be nice to factor out the S3/knox putBuffer error handling into a promise-returning function. We'd probably need to wrap the underlying errors or failure scenarios (non-200 status). Then the catch function will mostly be some logic to decide what to log, which is less fragile than deciding that we have indeed encountered an error and passing control to the appropriate continuation.

I think we ought to skip this for now and log it as an issue for later.

[hampelm]

@prashtx This is ready for the next round of review. I'm also going to test on my dev server.

[prashtx]

Other than the postgres note, LGTM.