Now that MemPoolOptions has a correctly-determined max_size member, perform sanity checks on that instead of re-determining the
options.
This concludes backport of core#25290
https://github.com/bitcoin/bitcoin/pull/25290/commits/d1684beabe5b738c2cc83de83e1aaef11a761b69
Depends on D15301
Note that some of the commits of this PR were not applicable, because they related to fee estimation or ancestor/descendant limits.