* eth/protocols, prp/tracker: add support for req/rep rtt tracking

* p2p/tracker: sanity cap the number of pending requests

* pap/tracker: linter <3

* p2p/tracker: disable entire tracker if no metrics are enabled

* eth/protocols/eth: split up the eth protocol handlers

* eth/protocols/eth: define eth-66 protocol messages

* eth/protocols/eth: poc implement getblockheaders on eth/66

* eth/protocols/eth: implement remaining eth-66 handlers

* eth/protocols: define handler map for eth 66

* eth/downloader: use protocol constants from eth package

* eth/protocols/eth: add ETH66 capability

* eth/downloader: tests for eth66

* eth/downloader: fix error in tests

* eth/protocols/eth: use eth66 for outgoing requests

* eth/protocols/eth: remove unused error type

* eth/protocols/eth: define protocol length

* eth/protocols/eth: fix pooled tx over eth66

* protocols/eth/handlers: revert behavioural change which caused tests to fail

* eth/downloader: fix failing test

* eth/protocols/eth: add testcases + fix flaw with header requests

* eth/protocols: change comments

* eth/protocols/eth: review fixes + fixed flaw in RequestOneHeader

* eth/protocols: documentation

* eth/protocols/eth: review concerns about types