Convert Payload from InputSupplier to ByteSource

This allows use of the modern Guava IO idioms, e.g., ByteSource.read()
instead of ByteStreams.toByteArray(InputSupplier).  Note that we must
replace calls to InputSupplier.getInput with overridden calls to
ByteSource.openStream to deal with InputSupplier.getInput checked
exceptions.

References JCLOUDS-410.