ovs-atomic: Introduce a new 'struct ovs_refcount'.

ovs-atomic: Introduce a new 'struct ovs_refcount'.

This is a thin wrapper around an atomic_uint.  It is useful anyhow because
each ovs_refcount_ref() or ovs_refcount_unref() call saves a few lines of
code.

This commit also changes all the potential direct users over to use the new
data structure.

Signed-off-by: Ben Pfaff <blp@nicira.com>