STATS: Add more hierarchy to counters.

Put error counters under node_index.
/err/<node-name>/<error-name>

E.g:
/err/ip4-lookup/Hash table collisions
/err/ip6-rewrite/Buffer allocation error

Error names are not globally unique, and this allows
for walking all errors for a particular node.

Put interface counters under the directory /if.
E.g. /if/rx

Put system counters under the directory /sys.
E.g. /sys/vector_rate

Change-Id: I5b794d16698f61bcb2063a8cd77a7c4ae36419b8
Signed-off-by: Ole Troan <ot@cisco.com>