Abins: Refactoring - caching, data flow

[ajjackson]

There are some significant quirks in the current implementation of data caching in Abins. At worst these lead to "spooky action at a distance" as it can be difficult to understand how data is passed between functions that rely on side-effects to change the cache state and read data from there.

I would prefer to resolve this before tying together Abins and Euphonic too tightly.

Therefore this is blocking #67 and #68