[validation/rpc] cache + use vsize calculated in PreChecks
Summary:
This is not only cleaner but also helps make sure we are always using
the virtual size measure that includes the sigop weight heuristic (which
is the vsize the mempool would return).
This is a b)rtial ackport of core#23381
https://github.com/bitcoin/bitcoin/pull/23381/commits/36a8441912bf84b4da9c74826dcd42533d8abaaa
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Subscribers: Fabien
Differential Revision: https://reviews.bitcoinabc.org/D12351