Reduce state memory

[greatect]

Intro The original katzenmint state keeps track of the received descriptors and documents for each epoch in memory. This is actually not necessary, as the current ones are the only thing relevant,. Old ones are saved in IAVL tree, which can still be retrieved.

Change

• Remove state.descriptors, state.documents, state.deferCommit
• Add state.prevDocument (to faciliate state.generateDocument())
• Use state.Get(storageKey(descriptorBucket, ...)) instead of state.descriptors[][]
• Use state.Get(storageKey(documentBucket, ...)) instead of state.document[]