[Linux] provide more memory statistics (peak rss mainly)

[giampaolo]

On Linux right now we read /proc/pid/statm to provide (rss, vms, shared, text, lib, data, dirty) memory metrics. /proc/pid/status provides much more metrics (ref: http://man7.org/linux/man-pages/man5/proc.5.html):

VmPeak:     131168 kB
VmSize:     131168 kB
VmLck:           0 kB
VmPin:           0 kB
VmHWM:       13484 kB
VmRSS:       13484 kB
RssAnon:     10264 kB
RssFile:      3220 kB
RssShmem:        0 kB
VmData:      10332 kB
VmStk:         136 kB
VmExe:         992 kB
VmLib:        2104 kB
VmPTE:          76 kB
VmPMD:          12 kB
VmSwap:          0 kB

Not all of them are useful but VmPeak definitively is. There are 2 problems though:

• /status provides all metrics of /statm but reading/parsing it is less efficient (around 25%)
• we may return these new metrics via memory_full_info() method but that relies on /proc/pid/smaps which requires different (higher) privileges

[pmav99]

isn't this a duplicate of #383 ?