cache: recycle existing cache with incremental updates

cache: recycle existing cache with incremental updates

Cache tracking has improved over time by incrementally adding/deleting
objects when evaluating commands that are going to be sent to the kernel.

nft_cache_is_complete() already checks that the cache contains objects
that are required to handle this batch of commands by comparing cache
flags.

Infer from the current generation ID if no other transaction has
invalidated the existing cache, this allows to skip unnecessary cache
flush then refill situations which slow down incremental updates.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>