* Problem: Outputs API doesn't respond with correct unspent/spent outputs
Solution: Fix fastquery such that embedded document is queried properly
* Problem: key order agnostic queries not implemented
Solution: get_spent queries embedded documents which respect key order. This is
not expected by the application hence the query should be altered to query any
kind of key order
* Problem: Mongo query for get_spent too complicated
Solution: Simplify query using $elemMatch
* Problem: No test for checking mixed spent outputs
Solution: Add test for `get_spending_transactions` to check that correct
matching is done when querying documents with multiple inputs
* Problem: tranasction ids not assert when getting spent outputs
Solution: assert tranasction ids
* Problem: we have unused and outdated fixtures
Solution: clean up fixtures and tests accordingly
* Problem: there are still unused fixtures
Solution: remove Merlin keys
* Problem: There are unnecessary markers
Solution: remove the tendermint marker for tests
* Problem: Source files contain no license info
Solution: Add comments with SPDX license info to source files
* Python 3 files don't need # -*- coding: utf-8 -*-
* Problem: 2 implementations of fastquery exist.
Solution: Remove the old deprecated implementation. Update the tests.
* Problem: There are still 3 outdated fastquery tests.
Solution: Fix the tests.
