docstore/all: Fix offset handling and extend test coverage

[bartventer]

This PR is a continuation of PR #3408 and includes additional changes:

• Fixes offset handling for RunGetQuery in docstore/memdocstore.
• Increases test coverage for documentIterator.Next in docstore/awsdynamodb. I'm not sure how realistic these test cases are, I can remove them if they don't add value.
• Extends test coverage for Limit/Offset related queries in docstore.

I also generated new replay files for awsdynamodb and gcpfirestore.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 73.26%. Comparing base (be1b4ae) to head (9d139c9).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3409      +/-   ##
==========================================
+ Coverage   73.12%   73.26%   +0.13%     
==========================================
  Files         113      113              
  Lines       14864    14869       +5     
==========================================
+ Hits        10870    10894      +24     
+ Misses       3219     3207      -12     
+ Partials      775      768       -7     

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