[DS-1352] - Itemimport replace issue

Tested code from Yin reported in JIRA issue 1352: https://jira.duraspace.org/browse/DS-1352
 
If the item belongs to more then one collection it's association is lost. Added method removeItemFromAllCollections to dspace-api which removes all item associations in collection2item.

This function is then used with Ying's code to correct the collections the item belongs to.