Add new per NUMA node memory statistics collector

It is sometimes useful to understand the distribution of free/occupied
memory between NUMA nodes to deal with performance problems. To do so,
add new meminfo_numa collector that enables exporting of per node
statistics along with unit and end-to-end tests for it.

Signed-off-by: Pavel Borzenkov <pavel.borzenkov@gmail.com>

collector/fixtures/sys/devices/system/node/node0/meminfo

@@ -0,0 +1,29 @@
+Node 0 MemTotal:       134182340 kB
+Node 0 MemFree:        53030372 kB
+Node 0 MemUsed:        81151968 kB
+Node 0 Active:          5456380 kB
+Node 0 Inactive:       59150184 kB
+Node 0 Active(anon):     691324 kB
+Node 0 Inactive(anon):   340456 kB
+Node 0 Active(file):    4765056 kB
+Node 0 Inactive(file): 58809728 kB
+Node 0 Unevictable:           0 kB
+Node 0 Mlocked:               0 kB
+Node 0 Dirty:                20 kB
+Node 0 Writeback:             0 kB
+Node 0 FilePages:      70170916 kB
+Node 0 Mapped:           894240 kB
+Node 0 AnonPages:        788196 kB
+Node 0 Shmem:             47860 kB
+Node 0 KernelStack:       34016 kB
+Node 0 PageTables:       143304 kB
+Node 0 NFS_Unstable:          0 kB
+Node 0 Bounce:                0 kB
+Node 0 WritebackTmp:          0 kB
+Node 0 Slab:            6654304 kB
+Node 0 SReclaimable:    4473124 kB
+Node 0 SUnreclaim:      2181180 kB
+Node 0 AnonHugePages:    147456 kB
+Node 0 HugePages_Total:     0
+Node 0 HugePages_Free:      0
+Node 0 HugePages_Surp:      0

collector/fixtures/sys/devices/system/node/node1/meminfo

@@ -0,0 +1,29 @@
+Node 1 MemTotal:       134217728 kB
+Node 1 MemFree:        39634788 kB
+Node 1 MemUsed:        94582940 kB
+Node 1 Active:          5604496 kB
+Node 1 Inactive:       71450592 kB
+Node 1 Active(anon):     590464 kB
+Node 1 Inactive(anon):   285088 kB
+Node 1 Active(file):    5014032 kB
+Node 1 Inactive(file): 71165504 kB
+Node 1 Unevictable:           0 kB
+Node 1 Mlocked:               0 kB
+Node 1 Dirty:               120 kB
+Node 1 Writeback:             0 kB
+Node 1 FilePages:      83579188 kB
+Node 1 Mapped:           864112 kB
+Node 1 AnonPages:        671932 kB
+Node 1 Shmem:             87580 kB
+Node 1 KernelStack:       31104 kB
+Node 1 PageTables:       124272 kB
+Node 1 NFS_Unstable:          0 kB
+Node 1 Bounce:                0 kB
+Node 1 WritebackTmp:          0 kB
+Node 1 Slab:            7020716 kB
+Node 1 SReclaimable:    4614084 kB
+Node 1 SUnreclaim:      2406632 kB
+Node 1 AnonHugePages:     90112 kB
+Node 1 HugePages_Total:     0
+Node 1 HugePages_Free:      0
+Node 1 HugePages_Surp:      0