Shorten the CTxMemPool initializer list using default initialization
for members that dont depend on the constuctor parameters.
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/20222 | core#20222]] [3/3]
https://github.com/bitcoin/bitcoin/pull/20222/commits/f15e780b9e57554c723bc02aa41150ecf3e3a8c9
Depends on D10842