Add two new sysctl vars to control the sync rate with the
main idea to reduce the rate for connection templates because
currently it depends on the packet rate for controlled connections.
This mechanism should be useful also for normal connections
with high traffic.

sync_refresh_period: in seconds, difference in reported connection
	timer that triggers new sync message. It can be used to
	avoid sync messages for the specified period (or half of
	the connection timeout if it is lower) if connection state
	is not changed from last sync.

sync_retries: integer, 0..3, defines sync retries with period of
	sync_refresh_period/8. Useful to protect against loss of
	sync messages.

	Allow sysctl_sync_threshold to be used with
sysctl_sync_period=0, so that only single sync message is sent
if sync_refresh_period is also 0.

	Add new field "sync_endtime" in connection structure to
hold the reported time when connection expires. The 2 lowest
bits will represent the retry count.

	As the sysctl_sync_period now can be 0 use ACCESS_ONCE to
avoid division by zero.

	Special thanks to Aleksey Chudov for being patient with me,
for his extensive reports and helping in all tests.
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Tested-by: Aleksey Chudov <aleksey.chudov@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>