[p2p] Limit inv syncs with new peers

[jcnelson]

It's getting expensive to sync a peer inventory from scratch, and it's an increasingly fragile process (especially with stackerdb sync). The inv state for a peer should either be kept around after disconnect (so we don't have to re-sync it), or we should avoid doing full inv syncs in the first place.

Also, a stackerdb disconnect should not result in a whole-sale peer disconnect.

[blockstack-devops]

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