Define a DLocker interface for distributed locker

[ahrtr]

Also updated the contrib/lock example to demo how to use the DLocker.

For more background, please refer to the article by Martin Kleppmann.

• It should be an improvement to help users better use etcd as a distributed lock.
• It's also backward compatible. It won't break any existing applications

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Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 68.78%. Comparing base (18eb5c6) to head (bddc7a9).

❗ Current head bddc7a9 differs from pull request most recent head 9786c9e

Please upload reports for the commit 9786c9e to get more accurate results.

Files with missing lines	Patch %	Lines
client/v3/concurrency/mutex.go	0.00%	3 Missing ⚠️

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Additional details and impacted files

Files with missing lines	Coverage Δ
client/v3/concurrency/mutex.go	63.75% <0.00%> (+2.71%)	⬆️

... and 16 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #18779      +/-   ##
==========================================
+ Coverage   68.74%   68.78%   +0.04%     
==========================================
  Files         420      420              
  Lines       35512    35515       +3     
==========================================
+ Hits        24411    24430      +19     
+ Misses       9671     9659      -12     
+ Partials     1430     1426       -4     

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[ivanvc]

/test pull-etcd-verify

I want to check if it was a flake in govet.

[ahrtr]

/test pull-etcd-integration-1-cpu-amd64