runtime: make Playground timestamps change when the stream fd changes

The process reading the output of the binary may read stderr and
stdout separately, and may interleave reads from the two streams
arbitrarily. Because we explicitly serialize writes on the writer
side, we can reuse a timestamp within a single stream without losing
information; however, if we use the same timestamp for write on both
streams, the reader can't tell how to interleave them.

This change ensures that every time we change between the two fds, we
also bump the timestamp. That way, writes within a stream continue to
show the same timestamp, but a sorted merge of the contents of the two
streams always interleaves them in the correct order.

This still requires a corresponding change to the Playground parser to
actually reconstruct the correct interleaving. It currently merges the
two streams without reordering them; it should instead buffer them
separately and perform a sorted merge. (See
https://golang.org/cl/105496.)

Updates golang/go#24615.
Updates golang/go#24659.

Change-Id: Id789dfcc02eb4247906c9ddad38dac50cf829979
Reviewed-on: https://go-review.googlesource.com/105235


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Yury Smolsky <yury@smolsky.by>
Reviewed-by: Ian Lance Taylor <iant@golang.org>