Implement INFORMATION_SCHEMA.SCHEMATA, SHOW DATABASES; verify DB names

[JanJakes]

This PR reverts #203, where providing the DB_NAME was made optional, and implements additional checks for the correctness of the database name.

• Implements the INFORMATION_SCHEMA.SCHEMATA table to store the database name (allowing for at most one user database).
• Checking whether the current session database name is correct.
• Migration path from older versions.
• Implementing SHOW DATABASES.

[JanJakes]

@adamziel Here's the DB name enforcing logic put back & improved to check for correctness. Everyone's AFK now, so I may just merge this tomorrow.

[JanJakes]

I was further thinking about this solution, and there's one thing I don't like — the use of the global variable, since MySQL global variables are not per database but per the whole instance.

That made me think — what's the native way of storing database names in MySQL? It's the information_schema.schemata and SHOW DATABASES. I will update it to store the DB name there; it will be more MySQL-native and future-proof (e.g., for multi-DB support). We can now have the limitation that only information_schema and DB_NAME can be stored there.

[JanJakes]

I was further thinking about this solution, and there's one thing I don't like — the use of the global variable, since MySQL global variables are not per database but per the whole instance.

That made me think — what's the native way of storing database names in MySQL? It's the information_schema.schemata and SHOW DATABASES. I will update it to store the DB name there; it will be more MySQL-native and future-proof (e.g., for multi-DB support). We can now have the limitation that only information_schema and DB_NAME can be stored there.

I ended up changing this to:

• Implementing the INFORMATION_SCHEMA.SCHEMATA table to store the database name (allowing for at most one user database).
• Checking whether the current session database name is correct.
• Migration path from older versions.
• Implementing SHOW DATABASES.

I'm leaving it open for review, as this can wait for when we're back from AFKs.

[adamziel]

Why this collation?

[JanJakes]

It's the default in MySQL 8+. See also the last commit separately for some more context. The upside is more correct MySQL compatibility, the downside that such exports won't work on older MySQL and MariaDB - but that's already the case with MySQL 8 as well. The default on MySQL 5 seems to be 'latin1_swedish_ci', which is not great.