cleanup(libsinsp): add scanf formats and fixed size variables in stats.cpp

[LucaGuerra]

What type of PR is this?

Uncomment one (or more) /kind <> lines:

/kind cleanup

Any specific area of the project related to this PR?

Uncomment one (or more) /area <> lines:

/area libsinsp

Does this PR require a change in the driver versions?

No

What this PR does / why we need it:

Tiny refactor of the stats implementation:

• Use fixed size ints (uint64_t) when reading status files
• Use the corresponding format specifier for fixed size ints (guarding against compilation warnings)
• Remove an assert on a condition that may actually happen (it does when you run UTs, especially as non-root)

cc @incertum @jasondellaluce

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[poiana]

LGTM label has been added.

Git tree hash: d23df0ec67c70e144afb4280c0af211945f1fd3a

[Andreagit97]

/approve

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Andreagit97, jasondellaluce, LucaGuerra

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