Convert the data reading in getPdfManager to be asynchronous

[Snuffleupagus]

This is not only shorter, but (in my opinion) it also simplifies the code.

Note: In order to keep the five different BasePDFStreamReader implementations consistent, we purposely don't re-factor the PDFWorkerStreamReader class to support for await...of iteration.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.79245% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.81%. Comparing base (c574694) to head (3117bc2).

Files with missing lines	Patch %	Lines
src/core/worker.js	86.79%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #20662      +/-   ##
==========================================
+ Coverage   59.79%   59.81%   +0.02%     
==========================================
  Files         169      169              
  Lines       90158    90156       -2     
==========================================
+ Hits        53907    53928      +21     
+ Misses      36251    36228      -23     

Flag	Coverage Δ
unittestcli	59.81% <86.79%> (+0.02%)	⬆️

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