Incremented version number to 0.19.6.

Summary: Version number incrememted to 0.19.6 and updated release-notes.

Test Plan: make check

Reviewers: jasonbcox, Fabien, deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2920