Add a reasonable default startDate to the endpoints

[mcauer]

Currently if you do not specify startDate param, it will be set to 1970-01-01T00:00Z, which is especially for interval queries producing a lot of useless time bins.

We could:

• set a hardcoded default at endpoint level
• or adjust SQL Queries to directly use min(changeset_timestamp) from the projection to have a data driven approach
• or request the min timestamp once on application initialization and use this as default for the endpoints as min timestamp usually does not change