[FIX] modules: set initial values for multi-process signaling to None to avoid missing events

For fresh databases, the signaling sequences in the
database stays at 1 until the installation of the
first module. If several workers are hit for this
database before the first module is installed,
this database registry will be loaded with a signaling
sequence of 1. Since  was the default value, any
signal received by workers in this state was ignored 
because they thought the registry was previously
not loaded.
Using None to indicate an  sequence value
is more accurate and avoids this error

bzr revid: dle@openerp.com-20140116151157-3zlyrg48xqn2lkd0