Encode destination using Base58 in WalletDB

Summary:
Previously destinations were encoded with EncodeDestination, which
depends on user configuration. WalletDB needs consistent encoding.

Base58 encoding is used for backward compatibility.

Tests for Purpose, Name and DestData in CWalletDB

Test Plan: added tests

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D745