Simplify State API class/interface hierarchy after all in-memory implementations are dropped

State API / impl modules provide three state types (singleton, queue, and K/V) with several implementations for each (on disk, in memory). Now all in-memory implementations are no longer used, everything is stored on disk (in a virtual mega map). It opens possibility to clean up the modules, in particular get rid of a few classes, merge some classes (e.g. merge `WritableKVStateBase` and `OnDiskWritableKVState`), and maybe even remove some interfaces.

Start by mapping the State API and impl modules, then trace the singleton, queue, and K/V types and their on-disk implementations. Identify which classes and interfaces remain used after the in-memory implementations were dropped; done means the hierarchy is simplified without retaining obsolete abstractions.

Written by the indexing model from the issue text.