Support setField before setRecord in MUD and MODE

[ludns]

We can't assume a setRecord before setField (as per convo in #479); and hence should add support for creating a default row before setting a field in MODE and MUD network.
We should probably put the default values of each Solidity type in schema-type for now (and for MODE in the mode package before we move that stuff to a go folder in the schema-type package).