XOP-229: stunnel cache improvements

Raise limits to 22 stunnels, idle 5min, age 3h.
Discard cache index entries for endpoints with no tunnels.
Better hashtable initial capacities.
Debug-logging to enable calculation of time saved by cache.

Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>

Conflicts:
	http-svr/xmlrpc_client.ml

Signed-off-by: Jerome Maloberti <jerome.maloberti@citrix.com>