Add function to combine multiple hash digests.
Adjust default batch size for sources.
Add cutoff functionality to PCAP source.

VAST now tracks the payload bytes of connections. When packets from a given
connection reach the cutoff, the PCAP source discards them.

The cutoff feature requires tracking connection state. If the maximum number of
connections has been reached, random connection entries get evicted. Moreover,
if a certain amount of time passes, the flow table evicts entries older than a
given duration. The time between expiration passes is also configurable as a
function of wall clock time.

Relates #51.