MINOR: filters: Select legacy or instance model in lifecycle callbacks

MINOR: filters: Select legacy or instance model in lifecycle callbacks

Make flt_init(), flt_init_per_thread(), flt_check(), flt_check_instances(),
flt_deinit() and flt_deinit_per_thread() process a single configuration
model, chosen with flt_use_legacy_filter(): the legacy filter configurations
or the finalized filter instances.

Instances are only initialized, checked and released in instance mode.
Legacy configurations are still released unconditionally because rules may
keep referencing them in instance mode, but their callbacks are not run when
instances were selected.

Previously both lists were always processed. This prepares attaching the
instances to streams without running startup callbacks twice.