various improvements

* remove association of name/metriccontext with metrics
* registration with metriccontext is now optional
* try to return as much memory to OS as possible for inspect
fixes for darwin for new metrics scheme
Merge branch 's-memfix'

* s-memfix:
  fixes for darwin for new metrics scheme
  various improvements

Conflicts:
	inspect/cpustat/cpustat_darwin.go
	inspect/memstat/memstat_darwin.go
	inspect/pidstat/pidstat_darwin.go