More expressive mint start and end terms

[casey]

Etching should be able to specify both start and end conditions. Start conditions determine when a mint starts, and end conditions determine when a mint ends.

For both start and end conditions, it should be possible to specify:

• A relative block count, i.e., a number of blocks after an etching is made
• A absolute block height

Additionally, mints should be able to specify a maximum supply, more than which cannot be minted. Maximum supply should be given in number of mints, not total supply, so we don't have to deal with partial mints.