election: fix the PD leader switch when etcd leader fake unchanged

[JmPotato]

What problem does this PR solve?

Issue Number: close #7780.

What is changed and how does it work?

Check the lessor ID and Raft term during lease keepalive to sense the etcd leader switch as soon as possible.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Release note

None.

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[codecov[bot]]

Codecov Report

Merging #7846 (06ec7dd) into master (d6d9fea) will increase coverage by 0.03%. Report is 2 commits behind head on master. The diff coverage is 81.81%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7846      +/-   ##
==========================================
+ Coverage   73.51%   73.55%   +0.03%     
==========================================
  Files         432      432              
  Lines       47913    47964      +51     
==========================================
+ Hits        35225    35278      +53     
- Misses       9642     9648       +6     
+ Partials     3046     3038       -8     

Flag	Coverage Δ
unittests	73.55% <81.81%> (+0.03%)	:arrow_up:

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[JmPotato]

/hold