Add flag to enable dst encoding using cashaddr

Summary:
Adds option -usecashaddr to enable encoding using cashaddr. Defaults to
false.

RPC tests always use the new cashaddr format.

Test Plan: make check, rpc tests

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D695