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 about:

It is expected that a client will impose a higher load on a server when it first connects:  the client may need may need to catch up on transactions it has missed, or to get trust lines or transfer fees.

(Remove duplication and "the client"/"it has" agreement.)