Fix/manifest dev server

[glebtv]

What changed:

• Fixed Plugin: "manifest" dev mode to fetch http:////manifest.json instead of reading stale FsPath/manifest.json.
• Production mode still reads from filesystem.
• Dev helper behavior remains per-render/per-helper-call reload.
• Added regression tests for:
• dev manifest reads from HTTP, ignoring stale disk manifest
• prod manifest reads from disk
• dev asset helper refetches HTTP manifest on each call
• Updated dependency:
• github.com/pkg/errors v0.8.1 -> v0.9.1