Extensionify ServerKeyShare

[davegarrett]

ClientKeyShare was converted from a message into an extension. I suggest doing the same for ServerKeyShare. This would make the two consistent with each-other and work towards issue #184.

[ekr]

Consensus in interim is "let's do it".

[davegarrett]

Here's an early WIP:
davegarrett@66d2fb3

The client extension and server message already use the same struct, so just merging them into one general extension is relatively simple and better than two almost identical extensions.

[davegarrett]

Ok, PR #268 filed for this. Will probably need some changes, so feedback / drive-by reviews would be appreciated in that PR.