search: Add escape hatch DISABLE_HORIZONTAL_INDEXED_SEARCH
Created by: keegancsmith
Even if you are not using horizontally scaled search, your searches will go through a new code path. In case this causes issues we introduce an escape hatch which will fallback to the old behaviour. To use this an admin will configure the environment:
DISABLE_HORIZONTAL_INDEXED_SEARCH=true ZOEKT_HOST=indexed-search-0.indexed-search:6070
Part of https://github.com/sourcegraph/sourcegraph/issues/5725