tapdb: add SupplyTreeStore implementation

In this commit, we add a concrete implementation of SupplyTreeStore in tapdb. This is similar to the structs we used to manage the multiverse tree, which is scoped by proof type. Instead, the supply trees are scoped by group key.

We can use this to stage a series of updates for a group key (mint, ignore, burn), and insert into a supply tree in a single atomic transaction. We also expose methods to fetch each sub supply tree as well.

Author: Roasbeef