Fetch current_epoch via render_api

[Shubhamg13]

This PR targets to access the Scene's PipelineInfo using render_api.

Aiming render_api as only interface used to communicate with WebRender.
As of now render_api is already being used for most of communication except this one (and update ).

In this PR

1. pipeline_info is moved to SceneBuilderThread along with removed_pipelines.
2. Connecting PublishPipelineInfo to SceneBuilerThread.
3. Along with making this pipeline_info synchronous as earlier we need to rely on calling of renderer.update() to PublishPipelineInfo. Now using event loop of SceneBuilerThread it is synchronous.

Add-ons
4. FlushPipelineInfo is also connected to SceneBuilerThread as required.

[Jon]

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