Gateway can self-DoS when IPFS blocks are not found during sync

[cdata]

It is possible to cause a gateway to self-DoS if there are missing blocks during a sync. The repro case is to have valid-looking link records that are not available in IPFS at the time that an address book is first sync'd to a gateway. This causes the gateway to try to resolve those records from IPFS, in turn delaying the push part of a sync while IPFS block lookups timeout for a couple of minutes. In the mean time, the name record being published by the sync'ing client expires and cannot be published to the name system (and the sync'ing client itself may have been disconnected due to timeouts configured on an ingress or similar).