BUG/MINOR: converters: Store the sink in an arg pointer for debug() converter

BUG/MINOR: converters: Store the sink in an arg pointer for debug() converter

The debug() converter uses a string to reference the sink where to send debug
events. During the configuration parsing, this string is converted to a sink
object but it is still store as a string argument. It is a problem on deinit
because string arguments are released. So the sink pointer will be released
twice.

To fix the bug, we keep a reference on the sink using an ARGT_PTR argument. This
way, it will not be freed on the deinit.

This patch depends on the commit e02fc4d0d ("MINOR: arg: Add an argument type to
keep a reference on opaque data"). Both must be backported as far as 2.1.