LSIF: retention policy

Created by: chrismwendt

Currently, LSIF uploads will accumulate until lsif-server runs out of disk space.

A retention policy would prevent that from becoming a problem. Any other ideas?

Retention policy questions:

• What's the dimension? Time, number of uploads per repo, number of commits, total size, etc.
• How is this configured? Probably env var
• Do the in-memory caches need to get cleared?
• What kind of recommendations will we put in our docs?