Bug 796474 - [email] quasi-infinite scroll may hitch under OMTC

This pull request supersedes the previous one for this issue:
https://github.com/mozilla-b2g/gaia/pull/18345

This one is similar in concept, but the vscroll module no longer uses the larger NodeCache approach, and just uses a simpler, direct cache of message item nodes. The outside API to vscroll stayed the same, so the rest of the pull request is the same, except for message_list's loadNextChunk, where there is an extra guard around the requestGrowth call.

As the UI is the same, carrying the ux-review+ over from the previous pull request.