Have Escrow forward all gas to recipient

[nventuro]

One of the reasons why PullPayment is useful is that by using _asyncTransfer, the 'payment' is decoupled from any possible recipient contract logic, and any code executed on an Ether transfer (e.g. a fallback function) will only run once withdrawPayments is called.

However, said function calls Escrow.withdraw, which does a transfer to the recipient. This means only 2300 gas is forwarded, which is only enough to e.g. log an event. This severely limits the usefulness of PullPayment (and Escrow): we should change this transfer for a call, along with the required checks (e.g. make sure the call actually succeeded, etc.).

[nventuro]

We've been discussing this a bit, and it's not entirely clear whether or not this could inadvertently create unsafe reentrant scenarios, so we're pausing development on this one until we have a clearer idea of the consequences of our decision.