Move-tables: Lag measurement mods

[zacharysierakowski]

Closes https://github.com/github/database-infrastructure/issues/8208

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Related issue: https://github.com/github/database-infrastructure/issues/8208
Related issue (public): #1681

Further notes in https://github.com/github/gh-ost/blob/master/.github/CONTRIBUTING.md
Thank you! We are open to PRs, but please understand if for technical reasons we are unable to accept each and any PR

Description

This PR fixes up the status message. Specifically, it addresses the "lag" measurement emitted in move-tables mode - where there is no heartbeat logic running.

• An applier-side "last applied event timestamp" updated after each successful DML batch.
• A derived "writer lag" metric exposed in: the periodic status line in move table mode (replacing the heartbeat-lag field), the hooks environment as GH_OST_BINLOG_WRITER_LAG_SECONDS (§1.7).
• An idle rule on the streamer: if no events have arrived for the configured heartbeat interval, bump the timestamp to now so silence is not interpreted as falling behind.

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

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