Parallel index build

[syvb]

No description provided.

[tjgreen42]

Done with initial pass. I didn't see any test-related changes: we need full unit test coverage both with and without parallelism enabled. Hard-coded limits (like # tuples in base relation => parallel vs not) will need parameterization for this purpose, otherwise unit tests will just revert to serial mode.

[syvb]

The parallel build logic I implemented doesn't work on PG13 - I've removed the CI build for it in parallel build (because it doesn't compile), and it's going to be EOL in a few months so I think it's fine to just remove PG13 support?

[tjgreen42]

Getting close. Besides issues mentioned below, we need some documentation updates to the README as part of this PR.

[tjgreen42]

The parallel build logic I implemented doesn't work on PG13 - I've removed the CI build for it in parallel build (because it doesn't compile), and it's going to be EOL in a few months so I think it's fine to just remove PG13 support?

Did you want to disable PG13 still? We probably should anyway.