Postgresql dependency query speedup

[patrickgoss]

Closes #1193

split the relationship CTE in order to speed the dependency query on large postgres dbs

[drewbanin]

Hey @patrickgoss - I like to create issues for every PR that we merge. This helps at release time when I need to write up the Changelog. Can you provide us with a brief summary of why you made this change, and how this change addresses the problem you're having? I can backfill an issue with that info :)

I think this looks pretty good, but I'd like to kick off the whole test suite (happy to do that) and give it some functional testing as well. Would also love if @beckjake could take a quick look at this

[patrickgoss]

Should resolve #1193

I've ran the test suite according to 'CONTRIBUTING.md', though it would be good to have someone else double check. I also tested to see if dbt run would work again on our database and it did!

[beckjake]

I've triggered the tests, assuming they pass this looks great to me! This will create some merge conflicts, but it will be easy to resolve, so let's get this in before #1145

[drewbanin]

Merging this - thanks @patrickgoss :)