stats: python vpp_stats rewrite to access stat segment directly

stats: python vpp_stats rewrite to access stat segment directly

This module implement Python access to the VPP statistics segment. It
accesses the data structures directly in shared memory.
VPP uses optimistic locking, so data structures may change underneath
us while we are reading. Data is copied out and it's important to
spend as little time as possible "holding the lock".

Counters are stored in VPP as a two dimensional array.
Index by thread and index (typically sw_if_index).
Simple counters count only packets, Combined counters count packets
and octets.

Counters can be accessed in either dimension.
stat['/if/rx'] - returns 2D lists
stat['/if/rx'][0] - returns counters for all interfaces for thread 0
stat['/if/rx'][0][1] - returns counter for interface 1 on thread 0
stat['/if/rx'][0][1]['packets'] - returns the packet counter
                                  for interface 1 on thread 0
stat['/if/rx'][:, 1] - returns the counters for interface 1 on all threads
stat['/if/rx'][:, 1].packets() - returns the packet counters for
                                 interface 1 on all threads
stat['/if/rx'][:, 1].sum_packets() - returns the sum of packet counters for
                                     interface 1 on all threads
stat['/if/rx-miss'][:, 1].sum() - returns the sum of packet counters for
                                  interface 1 on all threads for simple counters

Type: refactor
Signed-off-by: Ole Troan <ot@cisco.com>
Change-Id: I1fe7f7c7d11378d06be8276db5e1900ecdb8f515
Signed-off-by: Ole Troan <ot@cisco.com>