Don't recompute the root on every update; only on commit

[musalbas]

We should pull @roysc's rewrite of smt.go upstream here (https://github.com/vulcanize/smt), as it provides a substantial and well-written performance improvement by not recomputing the root on every update, that also produces the same roots as this smt.

In order to do so we need to first:

1. Re-implement deepsubtree;
2. Re-add value storage.

Given that the Cosmos SDK store v2 may be abandoned, we are looking into resuming the work of integrating SMT into store v1 (celestiaorg/cosmos-sdk#12) by @tzdybal. The way that we re-add value storage will depend on the requirements of store v1 and in particular its requirements for historical state access, rollback, and pruning, so this needs to be analyzed first.