Merge #14626: Select orphan transaction uniformly for eviction

Summary:
7257353b93 Select orphan transaction uniformly for eviction (Pieter Wuille)

Pull request description:

The previous code was biased towards evicting transactions whose txid has a larger gap (lexicographically) with the previous txid in the orphan pool.

Tree-SHA512: e35f700aea5ed79d1bc57f64bffcb623424b40156fd0a12f05f74f981a8aa4175d5c18d042989243f7559242bdf1d6d720bcf588d28f43d74a798a4843f09c70

Backport of Core PR14626

Test Plan:

ninja
ninja check-all

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6488