codeintel: Add envvar to disable janitor process

Created by: efritz

As we migrate the Cloud instance off of the current legacy PVC, we'll need to disable some reconiciliation that the precise-code-intel-bundle-manager does between Postgres and disk. This introduces a flag that can be set to disable this process for a short time.

See https://docs.google.com/document/d/1ATRCW9xUW6aEldpnm2QPCuJqRgdGxEgKU13c3KQv2mQ for notes. Disabling this process should not affect anything, but may increase disk pressure and increase the number of "unreachable" Postgres records while it's disabled. This should only really matter if it's disabled for a matter of days.