Implement missing TURN attributes required for server-side processing

[arkgil]

Some of the TURN attributes which are not implemented yet must be validated by the server. Currently when Fennec encounters message with such attributes it will crash, because Jerboa raises on unknown RFC (as per RFC, unknown comprehension required attributes indicate that the message should be dropped).

For start, we need to implement three of them:

• DONT-FRAGMENT - Encoding and decoding of DONT-FRAGMENT attribute #68
• RESERVATION-TOKEN - Encoding and decoding of RESERVATION-TOKEN attribute  #69
• EVEN-PORT - Encoding and decoding of EVEN-PORT attribute #70