feat: escrow detector class

[KolevDarko]

Detection of Escrow events from the graph

There is not separate payment network for Escrow because escrow is just an intermediary phase of the existing payment networks, but currently Escrow is only implemented for ERC20FeeProxy PN.

The way escrow detection works is we get the escrow events from the graph, together with our payment/refund events.

Then the balance object returned from getBalance function in our Payment detectors contains an additional array called escrowEvents.

There are changes in multiple files here because the return type of getTransferEvents() and getEvents() was previously just an array of payment events, but now it is an object with two arrays:
{ paymentEvents: [], escrowEvents: [] }

[coveralls]

Coverage remained the same at 87.842% when pulling a4afa0f on ivo-escrow-detector-class into 04d99ff on master.