[JIT Variant] Split multi-signal queries into many single-signal queries

[dshemetov]

A version of #646

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

Multi-signal queries complicate the JIT work by introducing a lot of iterator overhead. This avoids that by splitting them into many single-signal queries. A few notes:

• this branch makes non-derived signals completely avoid JIT code path, so its speed on non-derived signals should be identical to dev
• derived signals that rely on the same base signal make the same SQL query twice, so there's room for optimization here; I'm hoping that database caching makes that extra complexity not worth it
• just like [JIT Variant] Non-streaming Pandas approach #1014, the Docker image name is jit-multi-sql instead of the branch name because / is not allowed
• has some conflicts with Convert covidcast signal OR clauses to UNION #1021 that we'll need to address later

[dshemetov]

Going with the much cleaner approach in #1049.