Add average elapsed time for the last filter ran

This patch calculates the average elapsed time for the last filter ran
over a concrete pcap capture. It uses os.clock() to benchmark.

* src/pf.lua
* src/pf_test.lua

$ make test | grep 'AVG ET'
tc id 1 AVG ET 3.488372093023e-07
tc id 2 AVG ET 6.0465116279068e-07
tc id 3 AVG ET 1.0232558139534e-06
tc id 4 AVG ET 3.5348837209303e-06
tc id 5 AVG ET 3.2391304347826e-06
tc id 6 AVG ET 2.76e-06
tc id 7 AVG ET 2.48e-06
tc id 8 AVG ET 2.2934782608697e-06
tc id 9 AVG ET 3.054347826087e-06
tc id 10 AVG ET 0
tc id 11 AVG ET 0

$ for i in $(seq 1 10); do make test | grep 'AVG ET'; done | sort
[...]
tc id 1 AVG ET 3.2558139534884e-07
tc id 1 AVG ET 3.4883720930233e-07
tc id 1 AVG ET 3.4883720930234e-07
tc id 1 AVG ET 3.4883720930235e-07
tc id 1 AVG ET 3.4883720930236e-07
tc id 1 AVG ET 3.953488372093e-07
tc id 1 AVG ET 4.1860465116279e-07
tc id 1 AVG ET 4.6511627906976e-07
tc id 1 AVG ET 4.6511627906978e-07
tc id 1 AVG ET 6.0465116279074e-07
tc id 2 AVG ET 6.5116279069764e-07
tc id 2 AVG ET 6.7441860465116e-07
tc id 2 AVG ET 6.9767441860466e-07
tc id 2 AVG ET 7.2093023255809e-07
tc id 2 AVG ET 7.2093023255812e-07
tc id 2 AVG ET 7.2093023255814e-07
tc id 2 AVG ET 7.4418604651162e-07
tc id 2 AVG ET 7.6744186046517e-07
tc id 2 AVG ET 7.9069767441863e-07
tc id 2 AVG ET 9.0697674418603e-07
tc id 3 AVG ET 1.0232558139535e-06
tc id 3 AVG ET 1.0232558139535e-06
tc id 3 AVG ET 1.1627906976744e-06
tc id 3 AVG ET 1.4883720930233e-06
tc id 3 AVG ET 1.5348837209303e-06
tc id 3 AVG ET 1.7906976744186e-06
[...]