Sockets are added to the free list with .push() but they were
being removed with .shift().  This meant the sockets where being
removed in FIFO order, but this changes it to LIFO.  Since older
sockets may be closed based due to inactivity on the server it is
more likely that a socket that is recently used will be able to
successfully process the next request.

Rather than destroying the last used socket destroy
the oldest socket in the free list in push() on the
last recently used socket.

Add a test that ensures the HTTP agent reuses sockets
in a LIFO fashion rather than FIFO.