Change task_id to string

[yuwmao]

Changing task_id to string allows users to use more meaningful information as id.
In NuObject2.0, CM will use "timestamp:pg_id" as task_id for easy time based range query in its kvstore.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 31 lines in your changes missing coverage. Please review.

Project coverage is 55.69%. Comparing base (1a0cef8) to head (0e29dfc).
Report is 221 commits behind head on master.

Files with missing lines	Patch %	Lines
src/lib/replication/repl_dev/raft_repl_dev.cpp	0.00%	20 Missing ⚠️
src/lib/replication/repl_dev/raft_repl_dev.h	0.00%	7 Missing ⚠️
src/lib/replication/service/generic_repl_svc.cpp	0.00%	2 Missing ⚠️
src/lib/replication/service/raft_repl_service.cpp	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #758      +/-   ##
==========================================
- Coverage   56.51%   55.69%   -0.82%     
==========================================
  Files         108      109       +1     
  Lines       10300    12632    +2332     
  Branches     1402     1801     +399     
==========================================
+ Hits         5821     7036    +1215     
- Misses       3894     4840     +946     
- Partials      585      756     +171     

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[Besroy]

LGTM