Notable refactoring:
- Use stdlib "context" in place of "golang.org/x/net/context"
- Go-kit no longer wraps errors, so we remove the unwrap in transport_error.go
- Use MakeHandler when setting up endpoint tests (fixes test bug caught during
this refactoring)
Closes#1411.
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.
* Defining a concrete type for session tokens
* More rightish vc.IsLoggedIn()
* using type conversion instead of a method call
* include sessions in test viewer contexts
