Use const in COutPoint class
Summary:
Refactoring: all cases of using (uint32_t) -1 in COutPoint class are replaced with const; also all remaining instances of (UNSIGNED)-1 transformed to std::numeric_limits<UNSIGNED>::max() (by @practicalswift).
Backport of core PR14838.
Test Plan:
ninja check
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D5464