SafeSubReadStream is basically a SeekableSubReadStream that
re-seek()s the parent stream before each read(). That way, more than
one SafeSubReadStream of the same parent stream can be used safely
at the same time, at the cost of increasing IO.
Instead I moved the call to ReadStream::eos into SubReadStream::eos.
fuzzie pointed out that in certain cases this resulted in a lot of eos calls
before. Those were rather needlessly, since checking the eos bit of the
decorated stream in read does not give any advantage over checking it when
the client code really wants to test the eos flag.
