use delta queries for calendar events

Hacks the beta version call into the events api
when iterating through items, allowing us to run
delta-based queries for calendar events.

src/internal/operations/backup_integration_test.go

@@ -735,14 +735,13 @@ func (suite *BackupOpIntegrationSuite) TestBackup_Run_exchangeIncrementals() {
 				container2: {},
 			},
 		},
-		// TODO: not currently functioning; cannot retrieve generated calendars
-		// path.EventsCategory: {
-		// 	dbf: eventDBF,
-		// 	dests: map[string]contDeets{
-		// 		container1: {},
-		// 		container2: {},
-		// 	},
-		// },
+		path.EventsCategory: {
+			dbf: eventDBF,
+			dests: map[string]contDeets{
+				container1: {},
+				container2: {},
+			},
+		},
 	}
 
 	// populate initial test data