Improve connection speed

This is an umbrella task for features that improve connection speed and/or reduce number of roundtrips.

Currently connection with TLS 1.3 and PARSEC is done in the following steps:

1. S→C, welcome packet, short nonce ("scramble")
2. S←C, switch to TLS
3. S←C, TLS 1.3 Hello
4. S→C, TLS 1.3 Hello, ciphers, certificate
5. S←C, TLS 1.3, Handshake finished
6. S←C, username, password (native)
7. S→C, switch to PARSEC, long nonce
8. S←C, empty
9. S→C, ext-salt
10. S←C, password (parsec)
11. S→C, ok packet

That's four round-trips. One is TLS related, one is caused by wrong initial auth plugin used, one by ext-salt, and one to actually deliver the password to the server and get the auth status back. The firs three are an unnecessary overhead.

When all tasks below are done, we'll have 1-roundtrip 3-packet auth as in MySQL 3.23, but with full TLS and PARSEC.