Replaced nHashType with SigHashType class in wallet.cpp

Summary: Continue work replacing nHashType with SigHashType (see T61). Depends on D860.

Test Plan: Existing tests pass.

Reviewers: deadalnix, schancel, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D858