* eth/downloader: refactor downloader + queue

downloader, fetcher: throttle-metrics, fetcher filter improvements, standalone resultcache

downloader: more accurate deliverytime calculation, less mem overhead in state requests

downloader/queue: increase underlying buffer of results, new throttle mechanism

eth/downloader: updates to tests

eth/downloader: fix up some review concerns

eth/downloader/queue: minor fixes

eth/downloader: minor fixes after review call

eth/downloader: testcases for queue.go

eth/downloader: minor change, don't set progress unless progress...

eth/downloader: fix flaw which prevented useless peers from being dropped

eth/downloader: try to fix tests

eth/downloader: verify non-deliveries against advertised remote head

eth/downloader: fix flaw with checking closed-status causing hang

eth/downloader: hashing avoidance

eth/downloader: review concerns + simplify resultcache and queue

eth/downloader: add back some locks, address review concerns

downloader/queue: fix remaining lock flaw

* eth/downloader: nitpick fixes

* eth/downloader: remove the *2*3/4 throttling threshold dance

* eth/downloader: print correct throttle threshold in stats
Co-authored-by: Péter Szilágyi <peterke@gmail.com>