[MEDIUM] session counters: add conn_rate and sess_rate counters

[MEDIUM] session counters: add conn_rate and sess_rate counters

These counters maintain incoming connection rates and session rates
in a stick-table, over a period which is defined in the configuration
(2 ms to 24 days). They can be used to detect service abuse and
enforce a certain accept rate per source address for instance, and
block if the rate is passed over.

Example :
	# block if more than 50 requests per 5 seconds from a source.
	stick-table type ip size 200k expire 1m store conn_rate(5s),sess_rate(5s)
	tcp-request track-counters src
	tcp-request reject if { trk_conn_rate gt 50 }

	# cause a 3 seconds pause to requests from sources in excess of 20 requests/5s
	tcp-request inspect-delay 3s
	tcp-request content accept if { trk_sess_rate gt 20 } WAIT_END