Track Order Execution on closeEpoch and executeEpoch

[lucasvo]

Epochs are either fully executed in closeEpoch or in executeEpoch depending on whether 100% fulfillment of the orders is possible. If yes, closeEpoch increases the epoch count is increased by one in lastEpochExecuted. If not, execution happens at a later point and the subgraph triggers should not be run until executeEpoch is called.

When the epoch is executed, we should iterate over all investors with pending orders and update how much of their oder has been filled (the result of calcDisburse) and create a Tinlake Transactoin object that tracks the new investment.

When such an event occurs, we should create a Transaction object for each investor with a pending order as described in #143