Support "chunked list with cached height"

[etiennesegonzac]

One of the main gaia use case is probably going to be the following:

• we eventually fetch all the data
• we chunk the fetch and call reloadData() for every chunk
• we want to cache and return a complete fullHeight to let the user scroll at any point in the list from the get go

We should make this a very happy path :)

[wilsonpage]

Is this a job for <gaia-fast-list>?

[wilsonpage]

Perhaps it could accept a fullLength parameter to .configure() to make this easy?

[etiennesegonzac]

Is this a job for ?

I think it is :)
Might want to do some tweaks in fast-list, like cancel pending populate requests and all, but that's the idea.