Use record types instead of tuples in Rrd_reader

Signed-off-by: John Else <john.else@citrix.com>
Use record types instead of tuples in Rrd_writer

Signed-off-by: John Else <john.else@citrix.com>
Tag version 0.6.0

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