Send and subscribe transaction statuses endpoint

[AndriiDiachuk]

Closes: #745

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 64.28571% with 15 lines in your changes missing coverage. Please review.

Project coverage is 54.00%. Comparing base (7b58582) to head (0a99ee4).
Report is 11 commits behind head on master.

Files with missing lines	Patch %	Lines
access/grpc/grpc.go	67.50%	9 Missing and 4 partials ⚠️
access/grpc/client.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #772      +/-   ##
==========================================
+ Coverage   52.92%   54.00%   +1.08%     
==========================================
  Files          35       35              
  Lines        5580     5709     +129     
==========================================
+ Hits         2953     3083     +130     
+ Misses       2231     2199      -32     
- Partials      396      427      +31     

Flag	Coverage Δ
unittests	54.00% <64.28%> (+1.08%)	⬆️

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[peterargue]

let's call this nextIndex to it's clear what's stored

Suggested change

	messageIndex := uint64(0)
	nextIndex := uint64(0)