[p2p] advance PoX reward cycle synchronization only after both an inventory and downloader state-machine pass

[jcnelson]

Right now, the PoX sync watchdog doesn't verify that the block downloader to enter a quiescent state before proceeding to download the next PoX reward cycle's burnchain blocks. This can cause the node to process sortitions without having processed the anchor block, even though it exists and is available on the network, thereby leading to needless short-lived PoX reorgs. To address this (in the absence of #1805), the node should at least wait until the downloader has finished trying to fetch blocks for a given reward cycle before deciding to move to the next reward cycle.

[blockstack-devops]

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