Use the [SockJS Protocol](https://github.com/sockjs/sockjs-protocol) to handle
bidirectional communication instead of plain websockets. This allows
distributed queries to function in situations in which they previously failed
(Load balancers not supporting websockets, issues with Safari and self-signed
certs, etc.).
Also includes fixes to the JS message handling logic where slightly different
message delivery semantics (when using XHR) were exposing bugs.
Fixes#1241, #1327.