BUG/MINOR: init: Initialize random seed earlier in the init process

BUG/MINOR: init: Initialize random seed earlier in the init process

The random seed used in ha_random functions needs to be first
initialized by calling ha_random_boot. This function was called rather
late in the init process, after the init functions (INITCALLS) are
called and after the configuration parsing for instance which means that
any ha_random call in an init function would return 0. This was the case
in 'vars_init' and 'cache_init' which tried to build seeds for specific
hash calculations but ended up not being seeded.

This patch can be backported on all stable branches.