Caching of OSM data and/or geometries in flex backend

[joto]

With the flex backend it is possible to add the same OSM object into multiple tables. Currently the code doesn't do any caching. If the object is written to another table (i.e. if add_row() is called a second time) nodes in a way and members of a relation are re-requested from the database and the geometry is rebuilt.

We should have some kind of caching here that

• reuses the data we just got from the database
• reuses the geometry (if possible, depends on the geometry transformation used and parameters)