Track timestamp of each state in repl_dev_ctx.

[xiaoxichen]

With this we can easily get time diff of each given state, avoid doing the calculation/analysis from grep of logs.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 66.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 68.22%. Comparing base (1a0cef8) to head (05e7165). Report is 56 commits behind head on master.

Files with missing lines	Patch %	Lines
src/lib/replication/repl_dev/common.cpp	50.00%	5 Missing :warning:

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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #514       +/-   ##
===========================================
+ Coverage   56.51%   68.22%   +11.71%     
===========================================
  Files         108      109        +1     
  Lines       10300    10439      +139     
  Branches     1402     1400        -2     
===========================================
+ Hits         5821     7122     +1301     
+ Misses       3894     2642     -1252     
- Partials      585      675       +90     

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[zhiteng]

Do we still want this change? I'm cleaning stale/idle PRs.