Fix latency buckets with multiple lat cores

The bucket data of the last latency core was taken into account when
collecting the latency stats. We are now adding the bucket data for all
cores.
Also changed the error reporting when PROX instance is breaking the pipe
connection (e.g. when the dataplane network is overloaded).
Cleaning up some trailing spaces in pox_ctrl.py

Change-Id: I09ba01ac65e7e4e9ff03ad47da83aa4f83250a67
Signed-off-by: Luc Provoost <luc.provoost@intel.com>