Reorganize memory_state to avoid giant test module (#1258)

Prior to this PR `memory_state` defined and implemented functionality for three different types, `Chain`, `NonFinalizedState`, and `QueuedBlocks`. Each of these components will need a fair number of unit tests, and I realized that as its currently organized it would be difficult to organize the tests or at a glance figure out which tests are testing which components.

This PR changes the organization of `memory_state` such that each component it exports is defined in its own module. In follow up PRs each module will get its own test module, which will focus exclusively on unit tests for the item defined there-in.

- [Tracking Issue](#1250)