[Access] Change SendAndSubscribe endpoint's MessageIndex to start at 0

[AndriiDiachuk]

Closes: #6575
Related PR: onflow/flow#1520

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 7.69231% with 12 lines in your changes missing coverage. Please review.

Project coverage is 41.21%. Comparing base (899e12e) to head (299829d).

Files with missing lines	Patch %	Lines
engine/access/state_stream/backend/handler.go	12.50%	6 Missing and 1 partial ⚠️
access/handler.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6598   +/-   ##
=======================================
  Coverage   41.20%   41.21%           
=======================================
  Files        2052     2052           
  Lines      182191   182197    +6     
=======================================
+ Hits        75075    75088   +13     
+ Misses     100824   100818    -6     
+ Partials     6292     6291    -1     

Flag	Coverage Δ
unittests	41.21% <7.69%> (+<0.01%)	⬆️

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[Guitarheroua]

This also fixed the same issue for Accounts and Events. I know this is out of scope, but could you please also check MessageIndex for blocks to see if the implementation in all subscriptions is consistent?

Other than that, looks good to me.