ipn: add comment about thread-safety to StateStore

Updates #cleanup

Signed-off-by: Percy Wegmann <percy@tailscale.com>

ipn/store.go

@@ -72,6 +72,7 @@ func CurrentProfileKey(userID string) StateKey {
 }
 
 // StateStore persists state, and produces it back on request.
+// Implementations of StateStore are expected to be safe for concurrent use.
 type StateStore interface {
 	// ReadState returns the bytes associated with ID. Returns (nil,
 	// ErrStateNotExist) if the ID doesn't have associated state.