Re-lookup DNS names in HttpClient after connect failure

[TheBlueMatt]

@devrandom noted at #931 (comment) that we currently cache the dns resolution result in HttpClient and never re-look-up the name after repeated connection failure. We should cache the T: ToSocketAddrs type instead.

[devrandom]

As a specific use case, assume you have a kubernetes cluster, and bitcoind is on one of the nodes. If the nodes goes down and the service is migrated to another node, the client will experience a disconnect, and the service DNS entry will be repointed at the new node.