MEDIUM: init: use initcall for all fixed size pool creations

MEDIUM: init: use initcall for all fixed size pool creations

This commit replaces the explicit pool creation that are made in
constructors with a pool registration. Not only this simplifies the
pools declaration (it can be done on a single line after the head is
declared), but it also removes references to pools from within
constructors. The only remaining create_pool() calls are those
performed in init functions after the config is parsed, so there
is no more user of potentially uninitialized pool now.

It has been the opportunity to remove no less than 12 constructors
and 6 init functions.