fix: alway rejects write while downgrading region

[WenyXu]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

What's changed and what's your intention?

This PR makes the datanode immediately reject write operations while handling downgrade region instructions.

Previous behavior:
A region would only transition to a "downgraded leader" state after discovering its downgrade status through heartbeats, creating a potential timing gap.

New behavior:
Write operations are now immediately rejected when handling downgrade region instructions, and then flushes the memtable data.

With write operations rejected and memtable data flushed, the upgrade candidate process becomes faster since there's no data that would need to be replayed during the upgrade candidate region operation.

This PR also

• Changes the default region migration timeout from 30s to 300s.
• Changes the CLOSE_DOWNGRADED_REGION_TIMEOUT to 10s.
• Introduces metadata region last entry id to improve replay speed of metadata regions.

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

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[killme2008]

Question: When region downgrading fails due to a flushing timeout or other unexpected behaviors, what happens to this migration? Will it retry or abort?

[fengjiachun]

LGTM