detect/threshold: implement tracking 'by_flow'

detect/threshold: implement tracking 'by_flow'

Add support for 'by_flow' track option. This allows using the various
threshold options in the context of a single flow.

Example:

    alert tcp ... stream-event:pkt_broken_ack; \
        threshold:type limit, track by_flow, count 1, seconds 3600;

The example would limit the number of alerts to once per hour for
packets triggering the 'pkt_broken_ack' stream event.

Implemented as a special "flowvar" holding the threshold entries. This
means no synchronization is required, making this a cheaper option
compared to the other trackers.

Ticket: #6822.