DecayingSample takes time_point in ctor and uses time_point

Address JIRA RIPD-395

Switches a number of places in Resource::Logic to use abstract_clock::now()
which returns a time_point.  Unfortunately Resource::Logic tracks time
locally also, but with ints, not time_point.  So Resource::Logic uses a
delicate mix of abstract_clock::now() and abstract_clock::elapsed() with
this commit.  That inconsistency could be addressed in a second commit.
This isn't actually quite English.  :-)

/** Each Entry needs to know what time it is constructed at. */
or
/** Each Entry needs to know the time when it is constructed. */

Note that this uses JavaDoc style comments - because Doxygene (our documentation tool which we use sometimes and hope to use more) can extract these automatically.  Similarly, you should always end your sentences with a period in JavaDoc comments particularly, because they look really weird when taken out of the code without one...