[helpers] simplify concurrent_type structure

[KFilipek]

Since concurrent_type implements the same features as ranged, there is no need to do it twice.

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This change is 

[codecov-commenter]

Codecov Report

Merging #242 (5030c2a) into master (55f925a) will decrease coverage by 1.36%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #242      +/-   ##
==========================================
- Coverage   88.84%   87.48%   -1.37%     
==========================================
  Files           9        9              
  Lines         726      727       +1     
  Branches      127      127              
==========================================
- Hits          645      636       -9     
- Misses         48       57       +9     
- Partials       33       34       +1     

Flag	Coverage Δ
tests_gcc_debug_cpp17	87.48% <ø> (-1.37%)	:arrow_down:

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Impacted Files	Coverage Δ
pmemstream/src/region.c	79.06% <0.00%> (-8.44%)	:arrow_down:
pmemstream/src/libpmemstream.c	87.46% <0.00%> (+0.25%)	:arrow_up:

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