COLLECTIONS-770 When creating an iterator chain containing other iterator chains flatten the resulting internal object.

[stevebosman-oc]

Contributed on behalf of the @opencastsoftware open source team

I am slightly wary of opening up UnmodifiableIterator, but this seems to solve the performance degradation of nested IteratorChains seen in COLLECTIONS-770

[codecov-commenter]

Codecov Report

Merging #308 (2257274) into master (1677dac) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #308      +/-   ##
============================================
+ Coverage     85.87%   85.88%   +0.01%     
- Complexity     4676     4683       +7     
============================================
  Files           292      292              
  Lines         13469    13482      +13     
  Branches       1955     1959       +4     
============================================
+ Hits          11566    11579      +13     
  Misses         1326     1326              
  Partials        577      577              

Impacted Files	Coverage Δ
.../commons/collections4/iterators/IteratorChain.java	88.88% <100.00%> (+2.61%)	:arrow_up:
...s/collections4/iterators/UnmodifiableIterator.java	100.00% <100.00%> (ø)

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