Took into account of the reference of unmanaged instances

[CihatAltiparmak]

Adresses : #218 I have increased plugin_ref_count_ even when unmanaged instances is created. plugin_ref_count_ is only changed inside onPluginDeletion function and createRawInstance. onPluginDeletion is not executed for unmanaged raw instances. Thus, when plugin loader is deleted, shared library is not unloaded. In main branch, it directly unloads library in case only createUnmanagedInstance method is used as seen in the example in addressed issue.

I have also debugged with valgrind and it has below results.

My PR : https://gist.githubusercontent.com/DarkusAlphaHydranoid/7bde368143c6844ec427159e300cb2fc/raw/0ccecc5e9716fd85d315fc970c9724f96231424f/valgrind_log_class_loader_pr_217.txt

Your main branch: https://gist.githubusercontent.com/DarkusAlphaHydranoid/7bde368143c6844ec427159e300cb2fc/raw/0ccecc5e9716fd85d315fc970c9724f96231424f/valgrind_log_class_loader_commit_464fd14.txt

Looks like my branch has less memory leak.