hashmap: move reserve() from base class to public method

[lukaszstolarczuk]

This change is 

[codecov[bot]]

Codecov Report

Merging #937 (46f0862) into master (2df10e8) will increase coverage by 0.23%. The diff coverage is 92.30%.

@@            Coverage Diff             @@
##           master     #937      +/-   ##
==========================================
+ Coverage   95.70%   95.93%   +0.23%     
==========================================
  Files          48       48              
  Lines        6291     6279      -12     
==========================================
+ Hits         6021     6024       +3     
+ Misses        270      255      -15     

Flag	Coverage Δ
tests_clang_debug_cpp17	95.50% <92.30%> (+0.28%)	:arrow_up:
tests_gcc_debug	92.55% <90.90%> (+0.21%)	:arrow_up:

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Impacted Files	Coverage Δ
...ude/libpmemobj++/container/concurrent_hash_map.hpp	94.54% <92.30%> (+1.11%)	:arrow_up:
include/libpmemobj++/condition_variable.hpp	78.57% <0.00%> (+2.38%)	:arrow_up:
include/libpmemobj++/mutex.hpp	86.20% <0.00%> (+6.89%)	:arrow_up:

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