Minor: Don't recreate the Transformer for every record

[luandy64]

tap-loopreturns/tap_loopreturns/sync.py

Line 20 in 08a52ea

with Transformer() as transformer:

The issue with this is that the logs can get unusable because the Transformer tracks the set of all fields marked "not selected". And when the tap leaves the Transformer context manager, it prints that set.

The best approach I've seen to this problem is to create the Transfromer once per stream. And python has some interesting syntax that allows you to declare multiple contexts on one line:

with A() as a, B(some_arg) as b:
  a.do_thing()
  do_other_thing(b)