Add estimate size to dbwrapper

Summary:
As per title.

Depends on D507

Test Plan:

make check

D512 allow to test this better.

Reviewers: freetrader, CCulianu, #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: schancel

Differential Revision: https://reviews.bitcoinabc.org/D516