WL#4995: SSL Certificate validation

Currently, server certificates are not validated in Connector.NET 
(BUG#38700). We should add a possibility to optionally validate certificates, and 
maybe even validate them by default.

We will introduce the new connection string parameter 
"Ssl Mode" and corresponding MySqlConnectionStringBuilder property SslMode with 
possible values

1)None         : no encryption
2)Prefered     : use encryption if server supports SSL
3)Required     : deny connection if server does not support SSL
4)ValidateCA  : Check for valid certification authority in certificate
(host name mitmatch tolerated)
5)ValidateFull: Full validation.

The default value is "None".

This is modeled after Postgres client SSL support:
http://www.postgresql.org/docs/8.4/static/libpq-ssl.html (section 
30.17.3. "Protection provided in different modes")


Existing boolean parameter encrypt will be deprecated but still 
supported. "encrypt=true" in connection string will be handled the same as  
sslmode=prefered.

.Net client relies on OS support to validate certificates and does not 
implement its own certificate stores.