[qa] add cashaddr module, and fix send_big_transactions

Summary:
Previously, send_big_transactions sent the coins to OP_TRUE.
However, this means the coins did not show up in listunspent.
As a result, several tests are unable to transition to using
send_big_transcations over create_lots_of_big_transactions.

Test Plan: ./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1634