Time series support

[vr2262]

As suggested by @chrisnatali

As we learned from Formhub, there are people who would like to use the survey-submission paradigm to keep records of automated processes.

Should we support this? I'm not exactly sure how to handle this in a scalable way for arbitrary update frequencies.

[vr2262]

Background:

http://stackoverflow.com/a/4884384

https://news.ycombinator.com/item?id=8368509

Seems like people say SQL scales up just fine...

[chrisnatali]

Potential time-series sources include energy and credit tracking systems such as
massive_pumping_robot
and shared solar