exp/lighthorizon: Support order parameter in endpoints.

[Shaptic]

Currently, Horizon Lite ignores the order parameter. We need feature parity, so we need support for this parameter.

By default, we're behaving as if order=asc, in the sense that retrieving an account's transactions happens in "historical order," e.g. oldest first. We should behave differently if order=desc.

This task might actually be pretty involved as it may involve changes to the bitmap index to support a PrevActive().