MB-42668-FTS multi-collection index reports wrong number of docs

During the data insert operations of multicollection indexes,
we decorate the key with the collectionID as a prefix. Same key
decoration has to be applied during the data delete operations as well.

Change-Id: Ie2e755513bb3c1efb401a837da107d932ca3757a
Reviewed-on: http://review.couchbase.org/c/cbft/+/140095
Well-Formed: Build Bot <build@couchbase.com>
Tested-by: Sreekanth Sivasankaran <sreekanth.sivasankaran@couchbase.com>
Reviewed-by: Abhinav Dangeti <abhinav@couchbase.com>

pindex_bleve.go

@@ -2025,6 +2025,11 @@ func (t *BleveDestPartition) DataDelete(partition string,
 		return fmt.Errorf("bleve: DataDelete nil batch")
 	}
 
+	// need to apply the key decoration with multicollection indexes.
+	if t.bdest.bleveDocConfig.multiCollection() {
+		key = append(extras[4:8], key...)
+	}
+
 	t.batch.Delete(string(key)) // TODO: string(key) makes garbage?
 
 	revNeedsUpdate, err := t.updateSeqLOCKED(seq)

pindex_bleve_doc.go

@@ -138,6 +138,10 @@ func (b *BleveDocumentConfig) MarshalJSON() ([]byte, error) {
 	return json.Marshal(&tmp)
 }
 
+func (b *BleveDocumentConfig) multiCollection() bool {
+	return len(b.CollPrefixLookup) > 1
+}
+
 func (b *BleveDocumentConfig) BuildDocumentEx(key, val []byte,
 	defaultType string, extrasType cbgt.DestExtrasType,
 	extras []byte) (*BleveDocument, []byte, error) {