Use runtime.nanotime for duration tracking plus related restructuring

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.

nanotime_unsafe.go

@@ -0,0 +1,12 @@
+// +build !purego,!appengine,!js
+
+// This file contains the implementation of nanotime using runtime.nanotime.
+
+package puddle
+
+import "unsafe"
+
+var _ = unsafe.Sizeof(0)
+
+//go:linkname nanotime runtime.nanotime
+func nanotime() int64