Improve script efficiency

[jvimal]

Currently, mininet tracing uses kernel ftrace in sysfs to log trace output, by using "cat" and reading ASCII from kernel space, and flushing it to disk periodically (whatever "cat" does). A full blown ASCII is perhaps the least efficient representation of a tracing event, which is often just a few integers and maybe a 32 character string (for container/task name).

The "perf" utility (in linux-git/tools/perf) is built to efficiently log these tracing events. E.g. workflow:

# insmod mntracer.ko
# cd /tmp
# perf record -a -e mininet:* sleep 1
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.128 MB perf.data (~5589 samples) ]
# perf script
... shows same output