[Chronik] Optimize SpentByWriter using merge ops

Summary:
Speed up SpendByWriter significantly using merge ops: When chronikreindexing blocks 22000 to 221000, SpendByWriter::insert goes from 4.38s to 0.19s. When chronikreindexing the first 300000 blocks, it seems to shave off roughly 1700s, from 10059s to 8315.98s.

Depends on D14505.

Test Plan: ninja check-crates && ninja check-functional

Reviewers: Fabien, #bitcoin_abc, bytesofman

Reviewed By: Fabien, #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14430