Return on disk and DB independent size for the utxo set

Summary:
This achieve the same thing as core's PR10426 , however, it is not quite a backport of it as other part of the core codebase differs.

Depends on D502, D471

Test Plan:

../qa/pull-tester/rpc-tests.py blockchain.py

Reviewers: freetrader, CCulianu, #bitcoin_abc, sickpig

Reviewed By: #bitcoin_abc, sickpig

Subscribers: sickpig

Differential Revision: https://reviews.bitcoinabc.org/D507