Truncation of TransactionMemos & AccountModData

[taco-paco]

Layout of column doesn't allow for slot based cleanup. Since there's no way to map key -> slot. We need to:

1. Change layout adding slot as part of column format
2. Write migration script for clients where we map all keys to (0, current_key)

#332 (comment)_

[thlorenz]

Can we close thisj @taco-paco ?