flow: avoid adding twice same filter for node
flow: avoid adding twice same filter for node

Selecting node for period and requesting for
flow of them we could generated multiple time
the same filter. Use a map to generate one filter
per TID.

fixes: #276

skydive-project/skydive,lebauce/skydive,safchain/skydive,safchain/skydive,skydive-project/skydive,lebauce/skydive,safchain/skydive,lebauce/skydive,safchain/skydive,safchain/skydive,lebauce/skydive,skydive-project/skydive,safchain/skydive,lebauce/skydive,skydive-project/skydive,lebauce/skydive,skydive-project/skydive,skydive-project/skydive,lebauce/skydive,safchain/skydive,safchain/skydive,lebauce/skydive,skydive-project/skydive