Move backup & save related stuff from Buffer to SharedBuffer

Various methods of Buffer should be rather methods of SharedBuffer. This
commit doesn't move all of them to SharedBuffer yet, only those that
need to be moved to SharedBuffer in order to be able to request creating
or removing backups in other SharedBuffer methods.

internal/buffer/buffer.go

@@ -123,6 +123,8 @@ type SharedBuffer struct {
 
 	// Hash of the original buffer -- empty if fastdirty is on
 	origHash [md5.Size]byte
+
+	fini int32
 }
 
 func (b *SharedBuffer) insert(pos Loc, value []byte) {
@@ -223,7 +225,6 @@ type Buffer struct {
 	*EventHandler
 	*SharedBuffer
 
-	fini        int32
 	cursors     []*Cursor
 	curCursor   int
 	StartCursor Loc