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/protocol/bep_extensions.go

@@ -95,6 +95,10 @@ func (f FileInfo) SequenceNo() int64 {
 	return f.Sequence
 }
 
+func (f FileInfo) FileVersion() Vector {
+	return f.Version
+}
+
 // WinsConflict returns true if "f" is the one to choose when it is in
 // conflict with "other".
 func (f FileInfo) WinsConflict(other FileInfo) bool {