Current impl fetches pl, purge pl items, fetch RSS, inserts entries. If the RSS
fetch fails (remote server is temporary unreachable) the playlistitems for that
RSS are all dropped.
This has a side effect that RSS playlists cannot be determined from tracks (see
issue #1126) and thus cannot be deleted.
Simple fix to defer playlist item purge to the point that the RSS fetch is ok
and before the inserts
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.
