Continued Resource::Manger cleanup (JIRA RIPD-372)

 - Incorporates review comments from previous commit.
 - Increases Resource::Manager unit test coverage: verify a dropped consumer is blocked for a while.

I'm a strong believer in `const`.  It may not inform the reader but it informs the programmer and the compiler that we don't expect the value of the variable to change.  Effective C++ 3rd Edition Item 3: "Use const whenever possible".