Should check server SSL capability

It is also MySQL 5.6 compatibility problem for #32 .

MySQL 5.6.x Community Edition does not support SSL, so if the user has enabled SSL and the MySQL server is 5.6.x Community Edition, the user will receive an error message containing a handshake failure.

In this case, checking the MySQL server capabilities in HandshakeV10Message will be more useful than checking the MySQL server version.

My personal opinion is that for MySQL, SSL cannot support only enable/disable, and should support the following 3 modes:

• REQUIRED (high security): if server not support SSL, then throw a exception to channel (connection will be close by this exception)
• OPTIONAL (default mode): if server not support SSL, then disable SSL.
• DISABLED (plain mode): use plain connection without SSL.

R2DBC ConnectionFactoryOptions.SSL is a boolean option, I try treat true same as OPTIONAL, and append a SSL option Option.<Boolean>valueOf("sslRequired") for enable REQUIRED mode.

@mp911de Should I support optional SSL mode for MySQL 5.6 compatibility?

Another plan is just throw an exception and close the connection when the user has enabled SSL and the MySQL server does not support SSL. Then r2dbc-mysql still only has two modes, ENABLED or DISABLED. (Equivalent to ENABLED same as REQUIRED, and the OPTIONAL mode is removed)

[mp911de]

Not setting SSL to true means SSL config is undefined. We had a similar discussion at https://github.com/r2dbc/r2dbc-postgresql/pull/104

[mirromutth]

SSL_MODE has supported yet, and single flag SSL would been keep.