Initial plugin implementation

* refactor packages, flags, subscriptions
* DRY refactor builder tests
* use mockgen to generate mocks
* update README
* MODE=statediff no longer needed for unit tests
* simplify func names, clean up metrics
* move write params to service field
* sql indexer: confirm quit after ipld cache reset
  prevents negative waitgroup panic
* don't let TotalDifficulty become nil
* use forked plugeth, plugeth-utils for now

Makefile

@@ -0,0 +1,13 @@
+MOCKGEN ?= mockgen
+MOCKS_DIR := $(CURDIR)/test_helpers/mocks
+
+mocks: $(MOCKS_DIR)/gen_backend.go
+.PHONY: mocks
+
+$(MOCKS_DIR)/gen_backend.go:
+	$(MOCKGEN) --package mocks --destination $@ \
+		github.com/openrelayxyz/plugeth-utils/core Backend,Downloader
+
+docker: mocks
+	docker build . -t "cerc/plugeth-statediff:local"
+.PHONY: docker