* Use sync.Pool for objects in packet path.
Seeing cases of forwarding latency spikes that aling with GC.
This might be a bit overkill, but using sync.Pool for small +
short-lived objects in packet path.
Before this, all these were increasing in alloc_space heap profile
samples over time. With these, there is no increase (actually the lines
corresponding to geting from pool does not even show up in heap
accounting when doing `list` in `pprof`)
* merge
* Paul feedback
c3964ba2eb