Final code point size

[martinthomson]

I would like to propose that we choose a value for the final frame codepoint on the ACK_FREQUENCY frame that is from the 0x40-0x3fff range, so that the encoding takes two bytes. This is not going to be sent often.

For IMMEDIATE_ACK, a one byte coding might be useful to ensure that requesting acknowledgment is cheap.

[marten-seemann]

Related: #119

[mirjak]

We closed #199 as we didn't receive any further feedback from the group to change the code point. We can discuss again but it doesn't seem people have strong opinion and at some point we need to decide.

[LPardue]

Discussed in IETF 117, feeling in the room is to use a 1-byte code point for IMMEDIATE_ACK and a 2-byte code point for ACK_FREQUENCY.

[ianswett]

ACK_FREQUENCY already has a 2-byte codepoint, but IMMEDIATE_ACK needs one and both should be registered with IANA.