[Java] Fix UnboundedReaderAsSdfFn to avoid using unstarted unbounded reader.

[scwhittle]

Previously reportProgress could cache a reader for the initialRestriction of a source that was unstarted. If this was used for a subsequent read it could happen that the source is advanced and checkpointed without having start called.

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.33%. Comparing base (ac1c788) to head (5185a66).
Report is 19 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #34146      +/-   ##
============================================
+ Coverage     57.75%   59.33%   +1.58%     
- Complexity     1474     3282    +1808     
============================================
  Files           995     1166     +171     
  Lines        157686   178702   +21016     
  Branches       1076     3396    +2320     
============================================
+ Hits          91064   106041   +14977     
- Misses        64376    69256    +4880     
- Partials       2246     3405    +1159     

Flag	Coverage Δ
java	70.51% <ø> (+1.93%)	⬆️

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