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>

src/plugins/cnat/cnat_types.h

@@ -159,7 +159,6 @@ typedef struct cnat_timestamp_t_
 typedef struct cnat_node_ctx_
 {
   f64 now;
-  u64 seed;
   u32 thread_index;
   ip_address_family_t af;
   u8 do_trace;
@@ -173,7 +172,6 @@ extern uword unformat_cnat_ep (unformat_input_t * input, va_list * args);
 extern cnat_timestamp_t *cnat_timestamps;
 extern fib_source_t cnat_fib_source;
 extern cnat_main_t cnat_main;
-extern throttle_t cnat_throttle;
 
 extern char *cnat_error_strings[];