Use actix-ws for juniper_actix subscriptions

[tyranron]

Follows #1191, #1186

Motivation

See #1186 (comment):

2. Reimplement juniper_actix::subscritions via actorless actix-ws crate instead of the current actix-web-actors::ws one. The whole thing with actors cooperates very badly with juniper_graphql_ws state machine, introducing too excessive polling, which leads to panics and abnormal WebSocket closures instead of normal ones. It also gives a feeling of fighting with actors, trying to pair them with Stream/Sink, instead of easing it. So, the actorless actix-ws crate should go just fine, as we have in the juniper_warp crate.

Also, WebSocket connections are not closed normally in juniper_actix, due to panics:

Solution

Get rid of actix-web-actors::ws and use actix-ws crate instead.

Now, WebSocket connections are closing as expected:

Additionally

• Fixes some issues with the graphql-transport-ws implementation, regarding panicking for edge cases in polling.
• Adds juniper::http integration tests for graphql-transport-ws as well.