Make CMutableTransaction constructor explicit

Summary:
Silently converting to a CMutableTransaction will drop all caches
and should thus be done explicitly

This is extracted from Core PR13011 : https://github.com/bitcoin/bitcoin/pull/13011/commits/faab55fbb17f2ea5080bf02bc59eeef5ca746f07

We do not really care about the rest of the PR.

Depends on D3338

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3339