Update OSM layer boundary after import

Instead of updating the boundary of the OSM layer each time a node or
node data is inserted, update it once after all inserts have been
performed. This gives acceptable performance.

Minor changes:

- Clear cursor after inserting layer (create_required_layers.rb)
- Tidied initialize_database.pgsql
- Set imported timestamp on OSM layer
- Corrected end comments in server/routes/nodes/layer/put.rb
Merge branch 'dev' of github.com:foxdog-studios/citysdk-api into dev