Add openStream to ObjectLoader for big blobs

Add openStream to ObjectLoader for big blobs

Blobs that are too large to read as a single byte array should be
accessed through an InputStream based interface instead, allowing
the application to walk through the data stream incrementally.

Define the basic interface to support streaming contents, but don't
implement it yet for the file based backend.

Change-Id: If9e4442e9ef4ed52c3e0f1af9398199a73145516
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>