Split POIs across multiple days and store multiday trips in Datastore.

[eshika]

This CL does the backend work for multiday trips.

List of Changes:

1. Convert calcNumDays method from private to public so it can be used in TripServlet.java.
2. Input all locations into a DirectionsRequest, calculate optimal route, and get optimal ordering for all locations.
3. Split array of ordered locations into n arrays where n is the number of days in the trip. (A jagged array was used to store this split.)
4. Create a TripDay for each of the days in the trip using the locations subarrays.
5. Store all these TripDays in Datastore as children of the Trip being created.

Note: This CL does not include tests.