Ignore sighash when checking for DER encoding.

Summary:
This updates the IsValidSignatureEncoding such as it checks the signature without the sighashtype.

Depends on D1576 and D1572

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1577