Headers poc 4 - intermediate changes (#1125)

* Process BlockHeadersMsg

* Print invalidations

* Print reason for parent-child mismatch

* Fix off by one error

* Fix ExtendDown

* Replace tipLimiter with an LRU cache

* Retrieve 256 headers at a time

* Hard coding of tips, process new block hashes

* extract headers

* Write hard-coded headers into a file

* Add total difficulty

* Insert hard coded headers

* Log i

* Initiate download from hard-coded anchors

* Print the state of anchors

* Add hard-coded headers as tips

* Spare the peers

* Refine LRU caching of tips

* Print length of anchor trees

* Limit requests to 192 headers

* range error fix

* Print better, retain at least one tip in the LRU cache

* Skip duplicate line ends

* Throttle requests, display peer name

* Back off, intermediate tip queue

* Print block header timestamp

* Better logging

* Log less

* More logging of evictions

* Fix queue push

* Fix queue pop

* Exclude duplicate tips

* Fix tips logging

* Fix moving

* Add port flag

* Check headers

* Print extra

* Add mixDigest to serialisation

* Fix difficulty in hard-coded headers

* Avoid duplicate header requests

* Update peer characteristics on NewBlockMsg

* Remove backoff when peer responds

* Fix tests

* Fix lint

* Fix lint