Rewrite precise-code-intel-worker in Go

Created by: efritz

This is a rewrite of the precise-code-intel-worker in Go. This does not change the current entrypoints as we're not ready to switch to the new code yet. I am planning on manually testing requests against the old and new servers to ensure the behavior is the same during usage. A follow-up PR(s) will remove the old code and switch to the new code in build and deployment environments.

This new code needs to emit metric, add tracing to requests, and wrap errors properly before being considered ready for production.

Opening PR now for discussion as it's a large chunk of code. This effort is tracked in https://github.com/sourcegraph/sourcegraph/issues/9965.