Don't take a shared_ptr<> when the method does not share ownership.

As suggested by recent talks by:

Bjarne Stroustrup: 2015: Writing Good C++14:
video: https://www.youtube.com/watch?v=1OEu9C51K2A
slide: https://github.com/isocpp/CppCoreGuidelines/blob/master/talks/Stroustrup%20-%20CppCon%202015%20keynote.pdf

C++ Core Guidelines:
"F.7: For general use, take T* or T& arguments rather than smart pointers":
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#f7-for-general-use-take-t-or-t-arguments-rather-than-smart-pointers