Stop using CBase58Data for ext keys

Summary:
This is a partial backport of Core PR11372 : https://github.com/bitcoin/bitcoin/pull/11372/commits/ebfe217b15d21656a173e5c102f826d17c6c8be4

Depends on D3575

Test Plan:

make check

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3576