Split route data structure to storage (ro) / manipulation (rw) structures.

Split route data structure to storage (ro) / manipulation (rw) structures.

Routes are now allocated only when they are just to be inserted to the
table. Updating a route needs a locally allocated route structure.
Ownership of the attributes is also now not transfered from protocols to
tables and vice versa but just borrowed which should be easier to handle
in a multithreaded environment.