core: set default MaxIssuanceWindow when configuring from api
core: set default MaxIssuanceWindow when configuring from api

When configuring a new generator with corectl, you can set a
MaxIssuanceWindow. When using the api (dashboard), it settles on the
default. 

This resolves chain/chainprv#2068. 

Closes chain/chainprv#2179

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain