Add options what do with import conflicts

[marekcuchy]

While importing, it can happen that some of the imported elements (with equal OSM ID) are already in the DB. Currently, the importer ignores the new elements and logs them.

Add configuration so that the user can choose the conflict-resolving policy:

• Throw an exception
• Overwrite the data
• Keep old data, ignore new data (current policy)

Probably set default to throwing the exception.