Improve `database_api_impl::on_objects_removed` performance

In the lambda it looks for elements by iterating through a vector, which is inefficient.

Start at database_api_impl::on_objects_removed and inspect the lambda that searches the vector. Determine how object removal is represented and measure the current lookup behavior before choosing a less costly approach. Done means the removal handling preserves its existing behavior while avoiding the inefficient iteration; the issue names no test or benchmark to run.

Written by the indexing model from the issue text.