dataplane: copy buffer returned by pypcap

When reading packets pypcap always returns a pointer to the same statically
allocated memory, which will be overwritten the next time a packet is read. I
believe this is a bug in pypcap. The workaround is to make a copy immediately.
Merge into master from pull request #110:
dataplane: copy buffer returned by pypcap (https://github.com/floodlight/oftest/pull/110)