MINOR: filters: rework RESUME_FILTER_* macros as inline functions

MINOR: filters: rework RESUME_FILTER_* macros as inline functions

There is no need to have those helpers defined as macro, and since it
is not mandatory, code maintenance is much easier using functions,
thus let's switch to function definitions.

Also, we change the way we iterate over the list so that the calling
function now has a pseudo API to get and iterate over filter pointers
while keeping control on how they implement the iterating logic.

One benefit of this is that we will also be able to switch between lists
depending on the channel type, which is a prerequisite for upcoming
rework that split the filter list over request and response channels
(commit will follow)

No change of behavior is expected.