rpcdaemon: subscriptions, `newHeads` (#1359)

* fix `make grpc` on new checkouts

* update proto files

* add some stub

* prototype with fake events

* notifying about events

* pass events

* events are being sent

* transfer headers to filters

* create the “filters” struct

* implement new heads

* PoC of New Heads subscription

* fix keep alive

* fixups for the client

* add “type” to the event

* support header event type on client

* better stage refactor

* fixup for the eth backend

* fixups

* fix tests

* fix tests

* fix linters

* address comments

* remove unused log