[RFC][BC][FrameworkBundle] Force users to set "kernel.secret" to something unique

Bug fix: kinda*
Feature addition: no
BC break: yes
Symfony2 tests pass: yes
Fixes the following tickets: #6480
License of the code: MIT

This PR is to show different approach for "fix" suggested in #6480, as IMO there is no real point for "yet another listener" =)

This PR also introduces BC break for all users that used default value for `kernel.secret`, but IMO it's worth it.