Streams against MemoryFileSystems should not truncate

[kurtkilpela]

The implementation of MemoryFileSystem includes truncating files when sending #writeStream. It seems potentially accidental. This differs from the Disk-back variant which does not automatically truncate.

If time permits, GemStone's MemoryFileSystem should not automatically truncate. Instead, it should be as close to the Disk-backed variant as possible.