of1.2: set default arguments for FlowStatsRequest

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
of1.3: set default arguments for FlowStatsRequest

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
packet lib: add get_protocol API

get_protocols returns the list of protocols. This is useful for a
packet including the same protocol multiple times (e.g. tunneling such
GRE). However, it's a rare use case. Instead of
'get_protocols(hoge)[0]', let's do 'get_protocol(hoge)' simply.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>