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.

Tom said he found the emplace with the tuples hard to read and suggested a helper function.  It takes two helper functions (or possibly a templated helper function) because the parameters are different.  I don't feel strongly about either implementation.