Fix replication lag metric after initial replication

[rkistner]

Replication lag metric was introduced in #272 (not released yet).

This fixes an issue where the replication lag metric could keep on growing indefinitely after initial replication, if no other changes come in at that time.

The issue is that in the case of "Commit due to keepalive", the "oldest uncommitted change" was not cleared, even though we have nothing further to write. This fixes the issue.

Sample of what happened:

The sharp increase at 16:23 is when the initial snapshot is completed, and the drop at 17:20 is when another write was made to the source db, clearing the replication lag.

[changeset-bot]

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Latest commit: 8da22a4

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