MEDIUM: filters: Use macros to call filters callbacks to speed-up processing

MEDIUM: filters: Use macros to call filters callbacks to speed-up processing

When no filter is attached to the stream, the CPU footprint due to the calls to
filters_* functions is huge, especially for chunk-encoded messages. Using macros
to check if we have some filters or not is a great improvement.

Furthermore, instead of checking the filter list emptiness, we introduce a flag
to know if filters are attached or not to a stream.