If the close happens when next is being called, it can result in a race
condition where the current iterator gets set to nil after the initial
check.
This also fixes the finalizer so it runs the close method in a goroutine
instead of running it by itself. This is because all finalizers run on
the same goroutine so a close that takes a long time can cause a backup
for all finalizers. This also removes the redundant call to
`runtime.SetFinalizer` from the finalizer itself because a finalizer,
when called, has already cleared itself.
Daniel Moran
Stuart Carnie
Mark Rushakoff
Jason Wilder
Jonathan A. Sternberg