Offline: persist posts data - redux middleware approach

[rralian]

We have two potential approaches to caching... one in the calypso application via something like redux middleware, and one in a proxy service for the api. We should try both for this posts case and bump into the corners to see which approach seems the most immediately promising.

This task is to build persistence into the posts-list data structure.

[rralian]

This is tied to #2754, which is to persist redux state in general.

[rralian]

I'mma call this closed... we don't have redux persisting everywhere, but we have it persisting in some places (in pre-production anyway), and we can show how we're doing it.