Change tracking structures to use struct uuids

Change tracking structures to use struct uuids

In encaps.c, binding.c, and lport.c incremental processing
is aided by tracking entries by their ovsdb row uuids.
The original patch sets used pointers, which might lead
to errors if the ovsdb row uuid memory is released.  So,
use actual structures to hold the values instead.

Signed-off-by: Ryan Moats <rmoats@us.ibm.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>