rpc: Make gettxoutsetinfo/GetUTXOStats interruptible

Summary:

Make it interruptible, so that shutdown doesn't block for up to one
hour.

Backport of core PR19056 and PR19323 (bugfix).

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8645