* request: in the event of failure, try other IPs (#6761)

In the case where a name has multiple A/AAAA records, we should
try subsequent records instead of immediately failing when we have a
failure on the first IP address.

This significantly improves delivery success when there are network
connectivity problems affecting only IPv4 or IPv6.

* fix method call style

* request_spec: adjust test case to use Addrinfo

* request: Request/open: move private addr check to within begin/rescue

* request_spec: add case to test failover, fix exception check

* Double Addrinfo.foreach so that it correctly yields instances

* Add Request class with HTTP signature generator

Spec: https://tools.ietf.org/html/draft-cavage-http-signatures-06

* Add HTTP signature verification concern

* Add test for SignatureVerification concern

* Add basic test for Request class

* Make PuSH subscribe/unsubscribe requests use new Request class

Accidentally fix lease_seconds not being set and sent properly, and
change the new minimum subscription duration to 1 day

* Make all PuSH workers use new Request class

* Make Salmon sender use new Request class

* Make FetchLinkService use new Request class

* Make FetchAtomService use the new Request class

* Make Remotable use the new Request class

* Make ResolveRemoteAccountService use the new Request class

* Add more tests

* Allow +-30 seconds window for signed request to remain valid

* Disable time window validation for signed requests, restore 7 days
as PuSH subscription duration (which was previous default due to a bug)