core: count tx size in slots, bump max size ot 4x32KB (#20352)

* tests for tx size

* alow multiple slots transactions

* tests for tx size limit (32 KB)

* change tx size tests to use addRemoteSync instead of validateTx (requested in pool request).

* core: minor tx slotting polishes, add slot tracking metric

Co-authored-by: Michael Riabzev <RiabzevMichael@gmail.com>
Co-authored-by: Péter Szilágyi <peterke@gmail.com>