FastSerializer: We already know the attribute value

Change-Id: I6996e15f99b94b50c1903092d7424a96339ccfaf
Remove mutex from OSequenceOutputStream; it's intended to be fast

Make the caller responsible for using it wisely.
AFAICS it's used only in FastSaxSerializer, so far.

Change-Id: I50d48fd7f3e6cdeb9e4637868da213b8171e542b