When writing session to disk, ensure we do a flush

[bsclifton]

per discussion with @bridiver:

We'll want to use fs.fsync when saving session-store-1 to disk. We've had problems with this file being corrupted when a BSOD happens on Windows. The save that happens every 5 mins may not have been flushed.

Notes from the discussion:

• I think you have to use the low-level writeStream methods
• even writeSync doesn’t force a sync to disk
• the sequence should be create -> flush -> rename. The atomicity of rename on windows is questionable because of the method that node uses so we might want to consider patching it to use the documented atomic method
• node rename is definitely not atomic on windows and requires a second flush because it doesn’t use the MOVEFILE_WRITE_THROUGH flag
• create -> flush -> rename -> flush and then the file is guaranteed to be written to disk

[bsclifton]

We can use this API for doing the write:
brave/muon@2da2701

[bridiver]

muon.file.writeImportant(path, contents, cb(success))
The api handles serializing the saves on a per-filename basis and also blocks app.quit until the save is completed. No extra code to queue/block simultaneous writes is required and all writes happen on a separate thread.