Convert covidcast signal OR clauses to UNION

[LeonLu2]

closes #763

Prerequisites:

• Unless it is a documentation hotfix it should be merged against the dev branch
• Branch is up-to-date with the branch to be merged with, i.e. dev
• Build is successful
• Code is cleaned up and formatted

Summary

1. Refactored the QueryBuilder to convert covidcast signal OR clauses to UNION
2. Edited test_query.py accordingly to test filters changed in server code

[krivard]

this is your first major change to the code, so I was pickier than I would otherwise be.

a few logic confirmations which need to be addressed before approval, but mostly style suggestions

also: are the filter_X equivalents to the new alternative_filter_X functions being used anymore? If not we can just drop them.

[krivard]

Checking my understanding: this block will always execute, since conditions will always evaluate true: alternative_filter_strings wraps alternative_filter_values, and alternative_filter_values returns either a nonempty string ("False") or a nonempty list (a function of pair.signal)

Is that right?

[krivard]

I am reasonably satisfied, but I'm looping in Dmitry and George on this since it conflicts/interacts with a possible solution to a problem they're solving in an upcoming feature release for just-in-time processing.

[melange396]

The issue this was intended to address (#763) is no longer relevant, so i am closing this PR.