Rockskip: reduce round trips to DB

Created by: chrismwendt

• Replaces the tight loop of GetSymbol() with a single pl/pgsql function call
• Batches insertions of symbols

I expect this to improve speed in 25ms latency-to-DB environments by ~5x.

There are a lot fewer GetSymbol calls now and it's slightly faster even on my local machine:

Resolves https://github.com/sourcegraph/sourcegraph/issues/33751

Test plan

Automated tests