It is needed by the open order simulation alarm, which uses the indexation
timestamp.

It allows to have understandable accounting date on the invoice.

Maximum subscription date is closed on the 28.

Max day is 28.

Periodicity calculation should only be based on Hosting Subscription
periodicity, and not hardcoded in a script.

Limit as possible number of modifications on open order line.

Subscription start date should be based on beginning of the slap workflow, to
ensure compatibility with old data.

Use the state value only.
Correctly handle destroyed subscription which has never been associated to an
open order.

Try to reduce the number of documents created.
Always set a stop date on the open order line.

Fetching 1 element is faster than counting all.

Also commit test skeleton.