cnat: Fix throttle hash & cleanup

cnat: Fix throttle hash & cleanup

Type: fix

This fixes two issues :
- We used a hash to throttle RPC for adding fib entries,
but as we rely on a refcount, we cannot accept loosing an
entry, which could happen in case of a collision.
- On client cleanup we weren't freeing the fib entry correctly
which resulted in crashes when recreating an entry.
Added a test that ensures proper cleanup

Change-Id: Ie6660b0b02241f75092737410ae2299f8710d6b9
Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>