change initial authentication plugin in the server

configurable default authentication plugin for the server. "default" applies to the plugin name that the server uses for the first handshake packet, what plugin the server uses when no username is yet known

This was the wrong approach, there is no reason to make user to decide what plugin would be optimal in the initial packet. The idea is to reduce the number of roundtrips, the plugin in the initial packet should be the one that will most likely be used when the user name is known. And it's not user's job to estimate these probabilities. The server can track what auth plugins are used and automatically select the plugin with the highest likelihood.

• THD statically allocates 20 bytes for a scramble. Can be extended to 32 or malloc-ed every time
• MYSQL statically allocates 20 bytes for a scramble. Cannot be changed. The rest can go into the extension area.
• ed25519/parsec currently don't use the scramle from MYSQL (because it's too short) and request it from the server on every COM_CHANGE_USER. It might be the easiest to keep it that way, still storing the scramble in THD/MYSQL will save one round-trip for ed25519 (but not for parsec).