Change @@version and @@version_comment

[systay]

Description

This PR changes the @@version and @@version_comment values to include information about the Vitess version.
It also removed the recently added @@vitess_version.

The change includes a new file, version.go that contains very little. It's kept minimal like this to be easy for our release scripts to update the named version.

mysql> select @@version;
+------------------------------+
| @@version                    |
+------------------------------+
| 5.7.9-vitess-10.0.0-SNAPSHOT |
+------------------------------+

mysql> select @@version_comment;
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| @@version_comment                                                                                                                                                                        |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Version: 10.0.0-SNAPSHOT (Git revision d54b87c83 branch 'release-name-in-version') built on Wed Jan 20 20:42:46 CET 2021 by systay@Andress-MacBook-Pro.local using go1.15.5 darwin/amd64 |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

[askdba]

Should we not have a variable as vitess_version versus default mysql_version?
What we would like to identify the build that relates to Vitess Major Version.
So I suggest we keep version for MySQL vitess_version from Vitess.

+------------------------------+
| @@version                    |
+------------------------------+
| 5.7.9-vitess-10.0.0-SNAPSHOT |
+------------------------------+
1 row in set (0.00 sec)

mysql> select @@version_comment;
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| @@version_comment                                                                                                                                                          |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Version: 10.0.0-SNAPSHOT (Git revision bc4186242 branch 'release-name-in-version') built on Fri Jan 22 15:55:35 UTC 2021 by vitess@e355445e6586 using go1.15.6 linux/amd64 |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

Otherwise, this could be confusing for generic MySQL users expecting a different result.

[systay]

@askdba:
The reason I wanted to overwrite the @@version comes from seeing what Ubuntu does with their mysql installation:
5.7.27-0ubuntu0.18.04.1. Surely Vitess should be able to change the version string as much as Ubuntu can.

I don't think it's a good idea to simply pass on the underlying mysql version like we are doing today - they are not interacting with mysql of that version, they are interaction with Vitess which is very different. It already creates problems when users are running mysql 8.0, since some libraries/apps/orms start using features that Vitess does not support.

[deepthi]

@systay the docs need to be updated.
@Akilan1999 was a good citizen and added to the docs here: vitessio/website#668

[aquarapid]

Something for the eventual 10.0 release notes: the server version we advertise to connectors in on the protocol level has changed from "5.7.9-Vitess" to "5.7.9-vitess-10.0.0-SNAPSHOT" (or whatever); if anyone introspects this value, they might care.