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.

Saw all this going by and decided to pull up a lawn chair to watch.  When I finally figured out what all the discussion was about, I must admit to being a bit let down (maybe something better is on another channel - reaches for remote...).  Really, all this over two correct applications of const?  I must admit, I probably wouldn't have noticed that they were missing either.