QHash security fix (1/2): add global QHash seed

QHash security fix (1/2): add global QHash seed

Algorithmic complexity attacks against hash tables have been known
since 2003 (cf. [1, 2]), and they have been left unpatched for years
until the 2011 attacks [3] against many libraries /
(reference) implementations of programming languages.

This patch adds a global integer, to be used as a seed for the hash
function itself. The seed is randomly initialized the first time a
QHash detaches from shared_null.

Right now the seed is not used at all -- another patch will modify
qHash to make use of it.

[1] http://www.cs.rice.edu/~scrosby/hash/CrosbyWallach_UsenixSec2003.pdf
[2] http://perldoc.perl.org/perlsec.html#Algorithmic-Complexity-Attacks
[3] http://www.ocert.org/advisories/ocert-2011-003.html

Task-number: QTBUG-23529
Change-Id: I7519e4c02b9c2794d1c14079b01330eb356e9c65
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>