DynamoDB: Fix ScannedCount calculation for Limit-ed calls

[bblommers]

Fixes #7080

Our scan() implementation would iterate through the entire dataset, and only trim the results at the end.

This approach has multiple downsides:

• We're needlessly filtering/copying all items in the table
• The ScannedCount is wrong - because we're scanning the whole table, we're essentially saying that ScannedCount = len(table)

This PR changes the approach so that we only read the necessary items.

• we only start processing items when we've reached the ExclusiveStartKey (DynamoDB's pagination key)
• we immediately end processing when the result set has reached the desired size (either in length or in bytesize)

Note that query still follows the old approach, and presumably suffers from the same problems. We should rewrite that operation in the same way at some point.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (ad316dd) 95.87% compared to head (87f925c) 95.87%.

Files	Patch %	Lines
moto/dynamodb/models/table.py	97.61%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7085      +/-   ##
==========================================
- Coverage   95.87%   95.87%   -0.01%     
==========================================
  Files         834      834              
  Lines       82019    82036      +17     
==========================================
+ Hits        78638    78654      +16     
- Misses       3381     3382       +1     

Flag	Coverage Δ
servertests	35.82% <7.14%> (-0.01%)	⬇️
unittests	95.82% <97.61%> (-0.01%)	⬇️

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