add conntrack counter and conntrack max

[gregleroux]

Fixes #
#54

Summary of changes:
-add conntrack counter and conntrack max

How to verify it:

• load task with this collected metrics:
  "/intel/psutil/net/conntrackcount": {},
  "/intel/psutil/net/conntrackmax": {}

Testing done:

A picture of a snapping turtle (not required but encouraged):

[snapbot]

Can one of the admins verify this patch?

[kindermoumoute]

Few points:

• You need to gofmt your code (net.go doesn't verify go format).
• Medium test fail because it's using an old dependency of go-psutil, you need to update go-psutil dependency in glide.lock.