runtime.nanotime is significantly faster than time.Now. Use it unless
build tags prevent usage of unsafe.
Change LastUsedTime to LastUsedNanotime as a time.Time is no longer
available.
Add IdleDuration to make it more convenient to get the time a resource
has been idle.
Rename ReleaseIdle to ReleaseUnused to better indicate it is releasing
the resource without using it rather than that it is releasing a
resource acquired with AcquireAllIdle.
Refactor tests of usage tracking to not be intermingled with
AcquireAllIdle tests.
Jack Christensen