nat: nat44-ei/ed nat objects cleanup improvements

Improvements:
* Changed plugin disable call behavior from freeing data types
to calling appropriate nat plugin object delete calls for
pool addresses, mappings and interfaces.
* Added wrapper nat44_ei/ed_add_del_static_mapping function to
handle switch bound static mappings. This would also fix ip assignment
callback add/del bound static mapping issue preventing creation of the
mapping.

Fixes:
 * Fixed lingering object issue: some nat intertwined objects would
not free each other if not correctly deleted in proper order.
 * Fixed incorect order of FIB unlocks for pool addresses causing
syslog messages to use deleted FIBs in multiple VRF configuration.
 * Fixed incorrect value testing of flags instead of vrf_id for
multiple vrf configuration static mapping.

Type: improvement

Change-Id: I2743f7b1104b627bcc5ef937e3a50655313a26ea
Signed-off-by: Filip Varga <fivarga@cisco.com>