[Regression] Socket connection is always created

[w33ble]

Kibana version: 6.6+

Describe the bug:

The client side of the connection is always created when Kibana is loaded. This is a partial regression of the fixes in #24247.

This was introduced as part of #26068.

Steps to reproduce:

1. Open your debugger console
2. Load Kibana

Expected behavior:

The socket client should not be started unless an application requests it. I believe Canvas is still the only one that does.

Screenshots (if relevant):

[elasticmachine]

Pinging @elastic/kibana-app

[ppisljar]

1. canvas is no longer the only one requiring this, as every kibana visualization now uses the interpreter.
2. seemed to me that in canvas we created the socket on init, before anything would be using it (it was not initialized on first use, but before) ... i moved same behavrior to interpreter, where it might not make sense anymore.

[w33ble]

It loads the server on init, which is fine, and that's not what this issue is about. The issue is that load_browser_plugins just calls createSocket on import, so the socket is always initialized on the client. The socket client shouldn't try to connect until an app uses the socket (ie. apps should probably make their own calls to createSocket).

[timroes]

This is no longer relevant, since websocket support has been removed .. for now.