codeintel: Batch lsif store requests

Created by: efritz

When we stored data in SQLite, we designed the API to operate within one bundle. Once migrating the data to postgres, this is no longer necessary and we can request data from multiple bundles in one requset.

We should now attempt to batch as many queries as possible together within the codeintel API package to reduce the total number of queries (requesting multiple documents at once, etc).

This is an orthogonal effort to https://github.com/sourcegraph/sourcegraph/issues/15943.