rpc/blockchain: Use existing blockman in gettxoutsetinfo

Summary:
This is a backport of core#21767 [1/5]
https://github.com/bitcoin/bitcoin/pull/21767/commits/fc1c282845f6b8436d1ea4c68eb3511034c29bea

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11632