Replace CBitcoinSecret with {Encode,Decode}Secret
Summary: This is a partial backpor of Core PR11372 : https://github.com/bitcoin/bitcoin/pull/11372/commits/32e69fa0df8fc1cfc8ac4f8381bc54b8f33e1c38
Test Plan:
make check
Reviewers: #bitcoin_abc, markblundeberg
Reviewed By: #bitcoin_abc, markblundeberg
Subscribers: markblundeberg
Differential Revision: https://reviews.bitcoinabc.org/D3575