rgwlc: faithfully store lifecycle filters

Fix the bug where an empty lifecycle filter was stored
identically to one with an empty old-format prefix rule--
which caused the recovered rules to be formed incorrectly
as prefix rules.

Found by Kyle Bader and Tekton workflow.  Updated to not declare
virtual methods in LCFilter.

Fixes: https://tracker.ceph.com/issues/76806

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>