Update balance tracking logic (fees in v0.45.9)

[bryanchriswhite]

Background

As noted in #89, transaction fee amounts should be available in the respective event as of cosmos-sdk v0.45.8 (currently v0.45.9 due to dragonberry). #123 implemented an alternative approach to accounting for fee amounts in the context of balance change events which is sub-optimal by comparison (i.e. requires an additional round-trip to the DB).

Acceptance Criteria

The balance change handlers don't query the DB in order to construct new entities.