Add buffer_size option that is passed to epcap

buffer_size option can be defined for epcap application in the sys.config
file. It sets the internal buffer size for libpcap. By manipulating this
setting one can mitigate dropping packets by libpcap.
(Motivated by #283).
Improve document describing packet capture

The docs/packet_capture.md document has been extended with:
a. information on interpreting pcap logs,
b. information on using iperf to perform load test,
c. reference to the sample configuration file.
(Motivated by #283).