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_time.go

@@ -0,0 +1,13 @@
+// +build purego appengine js
+
+// This file contains the safe implementation of nanotime using time.Now().
+
+package puddle
+
+import (
+	"time"
+)
+
+func nanotime() int64 {
+	return time.Now().UnixNano()
+}