Fixes issue when performing an 'api/rescan' (results in RSS_SCAN_META scan type
in this module) that would result in a library_media_save() -> a readd even for
entries that already exist in db. This would result in dropping db record, incl
play_count etc instead of an update
2.10's mxmlDelete memleaks, and mxml is used in many parts of forked-daapd. So
to avoid that we ship upstream's fixed version of mxmlDelete and use that.
Makes it possible for the caller to request that a previous callback is
replaced, so we don't end up with more and more callbacks. Also add other
options for future use.
