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.

Consider losing "emplace".  The details of how you put the item into the table aren't important to the consumer of the API.

Consider also losing the "non-Admin" entirely - and having just addToTable and addToTableAdmin.