Clean message_count and last_message

Summary:
This commit clarifies the intended usage of message_count and
last_message. Additionally it changes the only usage of message_count
to using last_message instead, bringing the code further along the
intended usage.

This is a backport of core#19599

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10047