Seed node panics on oasis-node control queries

[ptrus]

Seed node currently panics on most oasis-node control methods, since it doesn't override most of the methods from the base backend (https://github.com/oasisprotocol/oasis-core/blob/master/go/consensus/api/base.go).

Some queries such as GetStatus() that do make sense for seed nodes should be implemented. For any others that don't make sense, make sure the node doesn't panic.