We don't care about the conversion from hash to JSON (that's an
ActiveModel::Serializer responsibility that is thoroughly tested) but
our logic so we can skip that step which only slows down tests.
It consistently reduced ~1.5s on my machine but it's still too slow to wait
~8.5s to get feedback from them.
Instead of relying on Spree::Order#outstanding_balance we make us of the
result set `balance_value` computed column. So, we ask PostgreSQL to
compute it instead of Ruby and then serialize it from that computed
column. That's a bit faster to compute that way and let's reuse logic.
We hide this new implementation under this features' toggle so it's only
used when enabled. We want hit the old behaviour by default.
