netfilter: conntrack: simplify event caching system

This patch simplifies the conntrack event caching system by removing
several events:

 * IPCT_[*]_VOLATILE, IPCT_HELPINFO and IPCT_NATINFO has been deleted
   since the have no clients.
 * IPCT_COUNTER_FILLING which is a leftover of the 32-bits counter
   days.
 * IPCT_REFRESH which is not of any use since we always include the
   timeout in the messages.

After this patch, the existing events are:

 * IPCT_NEW, IPCT_RELATED and IPCT_DESTROY, that are used to identify
 addition and deletion of entries.
 * IPCT_STATUS, that notes that the status bits have changes,
 eg. IPS_SEEN_REPLY and IPS_ASSURED.
 * IPCT_PROTOINFO, that reports that internal protocol information has
 changed, eg. the TCP, DCCP and SCTP protocol state.
 * IPCT_HELPER, that a helper has been assigned or unassigned to this
 entry.
 * IPCT_MARK and IPCT_SECMARK, that reports that the mark has changed, this
 covers the case when a mark is set to zero.
 * IPCT_NATSEQADJ, to report that there's updates in the NAT sequence
 adjustment.

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