Refactor PoS downloader (#3717)

* Remove blockHeight arg from FeedHeaderPoW

* Revert "Remove blockHeight arg from FeedHeaderPoW"

This reverts commit 9bf92921db11cd4e13386fcee29f30241d070fc6.

* Move PayloadMessage & ForkChoiceMessage to package engineapi

* RequestList instead of newPayloadCh & forkChoiceCh

* Introduce request status

* Send reply only when request status is New

* Move BeaconRequestList to HeaderDownload

* Don't SetFetching when PoS (Fetching means handling newBlockHashes)

* Merge Syncing & Synced into DataWasMissing

* Introduce SyncStatus

* onlyNew arg in WaitForRequest

* Move waitingForBeaconChain into RequestList

* Interrupt instead of skipCycleHack

* Introduce timeout for PoS headers

* Split downloadMissingPoSHeaders

* Move StartPoSDownloader into HeaderDownload

* Move Stopping interrupt to StartPoSDownloader

* Move stopping PayloadStatus back to HeadersPOS

* cleanUpPoSDownload

* Post-merge fix

* TestBogusForkchoice

* TestPoSDownloader

* requestStatus in attemptPoSDownload

* Broadcast in SetStatus

* Cosmetics

* attemptPoSDownload -> schedulePoSDownload

* Demote some logs to Trace