Code intel: Improve eviction strategy

Created by: efritz

Currently our eviction strategy is simple: Remove any commit that is

• older than PRECISE_CODE_INTEL_DATA_TTL (720h/~30d by default), and
• is not visible from the tip of the default branch

This isn't enough, as we need to now consider:

• indexes that exist as dependencies of another active index
• indexes that describe a named branch or tag, but is not itself tip (releases)

We need to start tracking additional information to more accurately determine what we can throw out.