[#4012] Performance: Use child_map to find tests for nodes in resolve_graph

[gshank]

resolves #4012

Description

The resolve_graph method that was implemented to support the build command caused a performance regression. This pull request switches to using the child_map to find tests for nodes.

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

[gshank]

The parent_map and child_map are not full-fledged members of the manifest class. Originally they were only created for writing out to the manifest.json. I needed some of the information in partial parsing so I built them when partially parsing. I've added another 'build_parent_and_child_maps' all to compilation.py. We ought to do some cleanup here and make them full-fledged attributes, but we'd also need to decide what to do about serializing and where to build them, etc, so I'm putting this pull request out for review without that extra work.

[jtcohen6]

I've tried this a bunch locally, and I haven't yet been able to break it. It's much, much faster.

I would like to see us add some more complex test cases (namely, tests with multiple parents) to the 069_build_test integration suite.

[isaacsantelli]

@gshank I submitted the original issue, this is the first time i've submitted a dbt issue, should I expect that the change will be in the dbt 0.22.0 or in an earlier build? Also thank you so much for the quick resolution either way :)