Improve performance

[prashtx]

Tweak survey data queries to take better advantage of an index.

Support more involved select fields for feature tiles, so we can avoid bloated UTFGrid data. Support multiple paths, so we can set up alternate domains and let clients issue more tile requests in parallel.

/cc @hampelm

[prashtx]

Made the pool bounds configurable, in case we'd like to test/tweak database performance more easily down the line.

[hampelm]

Thanks! Looks good to merge. I'll leave that to you so you can decide on the timing.