refactor subscribe func and stream mock object

[illia-malachyn]

Closes: #763

[illia-malachyn]

@peterargue cc

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 78.37838% with 24 lines in your changes missing coverage. Please review.

Project coverage is 56.59%. Comparing base (9f84549) to head (e2f4c9d).
Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
access/grpc/grpc.go	83.65%	14 Missing and 3 partials ⚠️
access/grpc/convert/convert.go	0.00%	5 Missing ⚠️
account.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #781      +/-   ##
==========================================
- Coverage   57.04%   56.59%   -0.46%     
==========================================
  Files          35       35              
  Lines        7391     7305      -86     
==========================================
- Hits         4216     4134      -82     
+ Misses       2673     2670       -3     
+ Partials      502      501       -1     

Flag	Coverage Δ
unittests	56.59% <78.37%> (-0.46%)	⬇️

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[peterargue]

much cleaner. added a few comments, but generally this looks good

[peterargue]

@illia-malachyn looks good. please merge master

[jribbink]

👌