support assisted discovery in oqgraph v3

OQGraph v3 tables have fixed structure, the manual says

The CREATE statement must match the format below - any difference will result in an error.

It would be much more user friendly to support assisted discovery and not force the user to specify the table structure at all. Then a user would only need to type, say,

CREATE TABLE oq_graph

ENGINE=OQGRAPH 

data_table='oq_backing' origid='origid' destid='destid';

Assisted discovery is easy, see federatedx.cc and sequence.cc for examples.