Reindex tickets on server start if no index exists

Reindex tickets on server start if no index exists

Check if tickets need to be reindexed when the server starts. This is the
case if no ticket index exists. In that case the ticket index is built.

This is done during the start of the `ITicketService`.

For this the interface of `ITicketService` needed to change. The `start`
method was defined abstract and the specific ticket services had to
implement it. None does any real starting stuff in it.
The `start` method is now final. It calls a new abstract method `onStart`
which the specific ticket services need to implement. In the existing
implementations I just changed `start` to `onStart`.