Prevent the heartbeat time from going back and causing leader lease invalid

[tangyuanzhang]

What type of PR is this?

• bug
• feature
• enhancement

What problem(s) does this PR solve?

Issue(s) number:

#5265

Description:

When the thread pool is blocked, step2 may also cause the heartbeat to fall back

How do you solve it?

Special notes for your reviewer, ex. impact of this fix, design document, etc:

Checklist:

Tests:

• Unit test(positive and negative cases)
• Function test
• Performance test
• N/A

Affects:

• Documentation affected (Please add the label if documentation needs to be modified.)
• Incompatibility (If it breaks the compatibility, please describe it and add the label.）
• If it's needed to cherry-pick (If cherry-pick to some branches is required, please label the destination version(s).)
• Performance impacted: Consumes more CPU/Memory

Release notes:

Please confirm whether to be reflected in release notes and how to describe:

ex. Fixed the bug .....

[wenhaocs]

Very impressive finding following the last finding of rocksdb writing stalling causing large lastMsgAcceptedCostMs_! Thanks for the contribution.