[chronik] rename the MerkleTree struct

Summary:
This diff renames the struct and makes a few comment more generic because I plan to reuse it transaction ids in addition to block hashes.
No change in behavior.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: tobias_ruck, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17522