M.P. onboarding issue

[stefanhengl]

Week 1-2

• Make sure you went through software engineer onboarding.
• Set up a backend development intro session with @stefanhengl
• Improve the team's onboarding docs

Week 2-3

Get started with some smaller tasks

• merging: set lock on index while merging zoekt#319
• indexserver: add debug command to tombstone repos zoekt#320
• webserver: fix stats calculation zoekt#321
• https://github.com/sourcegraph/sourcegraph/issues/33489

Week 3-4

• spike: cache matchTree zoekt#322
• spike: change regex engine zoekt#323

[sourcegraph-bot-2]

Heads up @jjeffwarner - the "team/search-core" label was applied to this issue.

[jjeffwarner]

@stefanhengl Please take a look at these tasks and determine which ones were closed and created a new story with the remaining items that we can pick up once we wrap up the tiger team work. We can close this one after the new one is created.

[stefanhengl]

All of these tasks are already tickets. I will close the tickets that are done and the rest can just stay open until we get around fixing them.