Improve performance of /periodsForInitiate

Currently, the /periodsForInitiate endpoint takes over 8 seconds when tested on the Malawi dataset. This is way more than is acceptable for a simple task that this endpoint does.

The retrieval of the periods from reference data takes only a couple of milliseconds, so most of the time is spent in the origin service.

Acceptance criteria:

• The endpoint takes less than 1 second when tested on the MW dataset