Writer needs to create file if it doesn't exist

Signed-off-by: John Else <john.else@citrix.com>
Re-raise errors from Reader.start and Writer.start

Signed-off-by: John Else <john.else@citrix.com>
Reader only needs to open and map the shared file RO

Signed-off-by: John Else <john.else@citrix.com>