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.
Start with the definition! 

Each server in a cluster has a list of IP addresses of end points that are imposing a significant load called Gossip, which is passed to other nodes in that cluster.  Gossip helps individual servers in the cluster to identify a set of IP addresses that are possibly unduly loading the entire cluster.