mbsync: use separate structure for old_data
mbsync: use separate structure for old_data

Assigning an attribute on Items doesn't really work here since we try to store
that value to the DB as a flexattr.

imsparsh/beets,MyTunesFreeMusic/privacy-policy,asteven/beets,marcuskrahl/beets,SusannaMaria/beets,shamangeorge/beets,jcoady9/beets,Freso/beets,Dishwishy/beets,m-urban/beets,shamangeorge/beets,beetbox/beets,mosesfistos1/beetbox,ibmibmibm/beets,parapente/beets,krig/beets,ruippeixotog/beets,lengtche/beets,xsteadfastx/beets,imsparsh/beets,mried/beets,sadatay/beets,shanemikel/beets,moodboom/beets,krig/beets,randybias/beets,multikatt/beets,Kraymer/beets,m-urban/beets,tima/beets,diego-plan9/beets,ruippeixotog/beets,arabenjamin/beets,sampsyo/beets,lightwang1/beets,Andypsamp/CODfinalJUNIT,jackwilsdon/beets,LordSputnik/beets,PierreRust/beets,dfc/beets,MyTunesFreeMusic/privacy-policy,madmouser1/beets,ttsda/beets,gabrielaraujof/beets,asteven/beets,Andypsamp/CODfinalJUNIT,drm00/beets,bj-yinyan/beets,ttsda/beets,lightwang1/beets,Andypsamp/CODjunit,ttsda/beets,beetbox/beets,marcuskrahl/beets,m-urban/beets,randybias/beets,ttsda/beets,swt30/beets,lengtche/beets,xsteadfastx/beets,mosesfistos1/beetbox,randybias/beets,drm00/beets,jcoady9/beets,pkess/beets,YetAnotherNerd/beets,sampsyo/beets,Andypsamp/CODjunit,swt30/beets,jcoady9/beets,madmouser1/beets,kareemallen/beets,Freso/beets,SusannaMaria/beets,Andypsamp/CODjunit,moodboom/beets,Kraymer/beets,sampsyo/beets,ibmibmibm/beets,sadatay/beets,andremiller/beets,jackwilsdon/beets,shanemikel/beets,artemutin/beets,tima/beets,beetbox/beets,Freso/beets,moodboom/beets,bj-yinyan/beets,madmouser1/beets,swt30/beets,gabrielaraujof/beets,ruippeixotog/beets,shanemikel/beets,Andypsamp/CODfinalJUNIT,gabrielaraujof/beets,Andypsamp/CODfinalJUNIT,MyTunesFreeMusic/privacy-policy,beetbox/beets,sadatay/beets,jmwatte/beets,artemutin/beets,andremiller/beets,multikatt/beets,ibmibmibm/beets,pkess/beets,imsparsh/beets,PierreRust/beets,ibmibmibm/beets,jackwilsdon/beets,andremiller/beets,multikatt/beets,mosesfistos1/beetbox,ruippeixotog/beets,LordSputnik/beets,mathstuf/beets,mried/beets,gabrielaraujof/beets,drm00/beets,bj-yinyan/beets,mathstuf/beets,kelvinhammond/beets,PierreRust/beets,marcuskrahl/beets,arabenjamin/beets,diego-plan9/beets,parapente/beets,sadatay/beets,jmwatte/beets,shanemikel/beets,arabenjamin/beets,Freso/beets,jmwatte/beets,Kraymer/beets,kelvinhammond/beets,lightwang1/beets,jackwilsdon/beets,m-urban/beets,YetAnotherNerd/beets,diego-plan9/beets,jmwatte/beets,lengtche/beets,mried/beets,artemutin/beets,multikatt/beets,Wen777/beets,parapente/beets,shamangeorge/beets,Dishwishy/beets,Dishwishy/beets,bj-yinyan/beets,marcuskrahl/beets,asteven/beets,arabenjamin/beets,mathstuf/beets,untitaker/beets,SusannaMaria/beets,LordSputnik/beets,kelvinhammond/beets,pkess/beets,sampsyo/beets,Wen777/beets,MyTunesFreeMusic/privacy-policy,swt30/beets,untitaker/beets,mosesfistos1/beetbox,kelvinhammond/beets,kareemallen/beets,dfc/beets,SusannaMaria/beets,diego-plan9/beets,Andypsamp/CODjunit,LordSputnik/beets,moodboom/beets,Wen777/beets,parapente/beets,asteven/beets,PierreRust/beets,madmouser1/beets,lengtche/beets,xsteadfastx/beets,krig/beets,dfc/beets,YetAnotherNerd/beets,kareemallen/beets,Andypsamp/CODfinalJUNIT,untitaker/beets,Kraymer/beets,xsteadfastx/beets,randybias/beets,tima/beets,untitaker/beets,jcoady9/beets,Dishwishy/beets,mried/beets,Andypsamp/CODjunit,lightwang1/beets,kareemallen/beets,imsparsh/beets,drm00/beets,mathstuf/beets,artemutin/beets,YetAnotherNerd/beets,tima/beets,shamangeorge/beets,pkess/beets,dfc/beets