Open Avro RMF files in shared-read mode on Windows
Open Avro RMF files in shared-read mode on Windows

By default Windows does not allow us to open a given
file more than once (this causes a sharing violation).
However, since we commonly do this with RMF files on
Linux/Mac, this leads to odd Windows-only issues. Be more
consistent across platforms by allowing multiple read-only
handles for a given RMF file.

salilab/rmf,salilab/rmf,salilab/rmf,salilab/rmf