Clean message_count and last_message
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 [[https://github.com/bitcoin/bitcoin/pull/19599 | core#19599]]

Test Plan: `ninja check-functional`

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

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

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