Enforce correct pool for cells

[JanKoelzer]

• Cells should not run in the "wrong" HandlerPool.
• Cells that depend on each other must be in the same HandlerPool.
• Rename to HandlerContext.
  See https://gist.github.com/phaller/a4fade7906bad95a61f24311ace62d66 for details.

[JanKoelzer]

You can use pool.mkCell() instead.