Update collations and make utf8mb4 default

[methane]

Description

Update collations table based on MySQL 8.0.12.

• Change default collation to "utf8mb4_general_ci" which is available from MySQL 5.5.
• Comment out collations can not be used for connection.

Fixes #839

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[shopee-jin]

gofmt please?

[methane]

Travis checks gofmt and no error is reported.

[julienschmidt]

Should we maybe make this version dependent instead?

According to https://www.monolune.com/what-is-the-utf8mb4_0900_ai_ci-collation/ the default collation is utf8mb4_0900_ai_ci since MySQL 8.0.1.

We already know the server version at the point where we send the collation.
So we could use utf8_general_ci for server version < 5.5.3, utf8mb4_general_ci for < 8.0.1 and utf8mb4_0900_ai_ci for 8.0.1 and newer.

[methane]

Should we maybe make this version dependent instead?

Personally speaking, I prefer stable behavior to "automatically changing default".
mysql client uses client's default, not server default.
User can specify collation explicitly.

According to https://www.monolune.com/what-is-the-utf8mb4_0900_ai_ci-collation/ the default collation is utf8mb4_0900_ai_ci since MySQL 8.0.1.

We already know the server version at the point where we send the collation.

I don't want to parse MySQL version string. MySQL version string syntax is not defined.
And MySQL is used by vary software, not only MySQL.
(e.g. MariaDB, PerconaDB, many MySQL proxy implementations)

So we could use utf8_general_ci for server version < 5.5.3, utf8mb4_general_ci for < 8.0.1 and utf8mb4_0900_ai_ci for 8.0.1 and newer.

Later is possible without parsing version string. Initial packet contains server's default collation id.
When it is 255 (utf8mb4_0900_ai_ci), it can be used by default.

But personally speaking, I don't like dynamic default. I prefer stable behavior.
User can choose other collation explicitly.

[shogo82148]

LGTM 👍

doesn't make sense.

[kaihendry]

If the collation default is not the same as my DB I'm connecting to, should I be worried? https://stackoverflow.com/questions/55998671/connecting-with-wrong-collation-to-a-mysql-server

[methane]

@kaihendry Your question is not specific to this driver. It's general to any MySQL driver.
Please don't ask such question on issue tracker.