94 of the open issues on sqlc are related to the analyzer. There are
many cases where the current analyzer produces false positives or
false negatives.

sqlx and some other projects have proven that it's possible to extract
query metadata from a running database.

This approach is a bit different, in that the database analysis is
layered on top of the existing query analyzer. We use the new analysis
to provide better type information and support a wider set of cases when
the existing analyzer fails.

Add a `contexts` key to exec.json to opt certain tests into or out of
database-backed analysis.

Fix many incorrect test cases that didn't run against an actual
database.