lib: remove useless checks, make functions inline on fast path

This patch:

* Removes a few useless checks on the fast path with the help of
  bt_object_put_no_null_check(), bt_object_get_no_null_check(),
  and bt_object_get_no_null_check_no_parent_check() when it's possible.

* Makes setting any frozen flag on the fast path only happen in
  developer mode.

* Makes fast path functions `static inline` (specific creating and
  recycling functions).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: If40b5f8dea9a085e8e30e25a9542767f3eb9815e