UnionInputStream: combines sequential InputStreams into one
The UnionInputStream utility class combines multiple sequential
InputStreams so they appear to the caller as a single stream with
no gaps. This can be used to concentate streams coming from multiple
independent HTTP connections (for example).
The companion unit test covers the class's full functionality.
Change-Id: I0676c7b5e082a5886bf0e8f43f9fd6c46a666228 Signed-off-by: Shawn O. Pearce <sop@google.com>