Added MAX_SCAN_IP_PAIRS

Added MAX_SCAN_IP_PAIRS

Thic commit allows psad memory usage to be constrained by restricting the
number of unique IP pairs that psad tracks via a new config variable
MAX_SCAN_IP_PAIRS.  This is useful for when psad is deployed on systems with
little memory, and is best utilized in conjunction with disabling
ENABLE_PERSISTENCE so that old scans will also be deleted (and thereby making
room for tracking new scans under the MAX_SCAN_IP_PAIRS threshold).