search: store repo ID in zoekt shards

Created by: keegancsmith

This is information we have wanted access to for a long time. Main use case is more efficient encoding of which repositories to search (int32 is much smaller than the name string). This won't be read, rather we want to ship this so we start storing the information.

Right now if the repo ID is not set it won't trigger a re-index. IE it does not affect "incremental" check. The intention is to not trigger a reindex of all shards. The following release we can enforce it is set, then the release after that assume it is available.

Zoekt Commits:

• https://github.com/sourcegraph/zoekt/commit/0f9dde4 indexserver: store sourcegraph repo ID in index
• https://github.com/sourcegraph/zoekt/commit/9ff81ec indexserver: Remove Error from IndexOptions

Best reviewed commit by commit.