[Draft] Changes for having replication thread per remote host

[Arun-LinkedIn]

[codecov-commenter]

Codecov Report

Attention: 46 lines in your changes are missing coverage. Please review.

Comparison is base (3995085) 70.71% compared to head (36416c8) 52.89%. Report is 1 commits behind head on master.

Files	Patch %	Lines
...om/github/ambry/replication/ReplicationEngine.java	17.85%	43 Missing and 3 partials :warning:

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #2715       +/-   ##
=============================================
- Coverage     70.71%   52.89%   -17.82%     
+ Complexity    11561     7915     -3646     
=============================================
  Files           830      830               
  Lines         70675    70719       +44     
  Branches       8499     8508        +9     
=============================================
- Hits          49978    37410    -12568     
- Misses        18074    30295    +12221     
- Partials       2623     3014      +391     

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[Arun-LinkedIn]

@snalli I haven't actually finished working on this pr. Want to do some WIP tests in EI to see if this change helps. That's why I put it in draft status and didn't add any reviewers :) But, thanks for initial comments!