Assign _all_ full seats

[mooori]

Currently full seats are assigned only until Config.seats_per_shard is reached for all shards. Excess full seats are ignored, see Config::collect_seats_for_shard().

To maximize the stake participating in validation, excess seats should be assigned as well. For that purpose, I would suggest to change full seat collection to do the same assignment via modulo as in Config::collect_partial_seats_for_shard().