Track future cashflows per asset

[hieronx]

• Any time the pricing of an asset changes (events: created, borrowed, repaid, priced, written off, ..)

• Use the cashflows RT API to store cashflows:

  type AssetCashflow @entity {
  id: ID! # pool id - asset id - cf timestamp
  asset: Asset! @index
  
  timestamp: Date!
  principal: BigInt!
  interest: BigInt!
  }

In a future ticket: actually link AssetTransactions to AssetCashflows.