Search pagination: preemptively fetch and cache results for users that are likely to come back soon

Created by: slimsag

After https://github.com/sourcegraph/sourcegraph/pull/4796 is merged, we have the opportunity to preemptively fetch the next page of results for a user which would make using the paginated API substantially faster.

This can be done using the UserID field in the search cursor (see its documentation for some caveats / edge cases we must handle for e.g. security reasons)