Enable asynchronous handling of HTTP-RPC (RIPD-390)
* Activate async code path
* Tidy up HTTP server code
* Use shared_ptr in HTTP server
* Remove check for unspecified IP
* Remove hairtrigger
* Fix missing HTTP authorization check
* Fix multisocket flags in RPC-HTTP server
* Fix authorization failure when no credentials required
* Addresses RIPD-159, RIPD-161, RIPD-390