backfill the network column of the exchange_routes table

[chadwhitacre]

Doesn't help to back route (#2779) if the route has an unknown network.

[chadwhitacre]

We have 1626 routes with unknown network.

We have 44993 exchanges (51%) linked to one or another of those routes.

gratipay::DATABASE=> select count(*) from exchanges e join exchange_routes er on e.route = er.id where network='unknown';
┌───────┐
│ count │
├───────┤
│ 44993 │
└───────┘
(1 row)

gratipay::DATABASE=> select count(*) from exchanges e join exchange_routes er on e.route = er.id;
┌───────┐
│ count │
├───────┤
│ 88622 │
└───────┘
(1 row)