Use C++11 member initializer in CTxMemPoolEntry

Summary:
This removes a bunch of boilerplate, makes the code easier to read.
Also, C++11 member initialization avoids accidental uninitialized
members.

Can be reviewed with the git option "--word-diff-regex=." or with "git
difftool --tool=meld".

This is a backport of core#23054

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12163