Improving relations hydration in ClosureHydrator

[msmakouz]

What was changed

1. Searching for a relation in the heap, instead of assigning a reference.
2. Assigning a Reference instead of loading a relation from db when the relation type is mixed in the entity.

[codecov]

Codecov Report

Merging #429 (d5c485e) into 2.x (8b85b83) will increase coverage by 0.22%.
Report is 10 commits behind head on 2.x.
The diff coverage is 85.71%.

@@             Coverage Diff              @@
##                2.x     #429      +/-   ##
============================================
+ Coverage     91.38%   91.60%   +0.22%     
- Complexity     1760     1766       +6     
============================================
  Files           112      112              
  Lines          4480     4505      +25     
============================================
+ Hits           4094     4127      +33     
+ Misses          386      378       -8     

Files Changed	Coverage Δ
src/Mapper/Proxy/Hydrator/ClosureHydrator.php	90.62% <80.00%> (+0.30%)	⬆️
src/Mapper/Proxy/ProxyEntityFactory.php	94.59% <100.00%> (+0.07%)	⬆️

... and 6 files with indirect coverage changes