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.
What you have there is using beast::uhash<>, which is not hardened.  To change it to hardened, change to:
ripple::unordered_map <std::string, Import, beast::hardened_hash<std::string>>.  I have no opinion on which is best here.  If the strings are supplied by untrusted code (over the net), we should use the hardened_hash.  Otherwise, I think either is fine.  The hardened_hash does not add significant expense.