Resistant Event Sub

[ggwpez]

Location: Package [backend/eth/glue], part of #19

Motivation: The Resistant Event Sub tracks the state of an event in the case of a chain reorganization.

Design: Analogous to #39 the design is very simple. It wraps an EventSub and accepts a finalityDepth. The necessary ChainReader is provided by the ContractBackend.

It tracks the status of an Event by subscribing to new block chain headers and then checking all events for finality.
Once an Event is final, it is emitted on the sink channel that the user passed to Read.

uml.txt

Testing: So far i am testing that final Events arrive even after reorgs. Still needs to be slightly extended.