lib/db: Add index to track locally needed files (#4958)

To optimize WithNeed, which is called for the local device whenever an index
update is received. No tracking for remote devices to conserve db space, as
WithNeed is only queried for completion.

lib/db/set.go

@@ -44,6 +44,7 @@ type FileIntf interface {
 	HasPermissionBits() bool
 	SequenceNo() int64
 	BlockSize() int
+	FileVersion() protocol.Vector
 }
 
 // The Iterator is called with either a protocol.FileInfo or a