Per-database options

MySQL/MariaDB uses the file DATADIR/dbname/db.opt to store
the database default character set and collation.
We could also store more per-database options in this file.
For example, a default storage engine would be convenient:

CREATE DATABASE db1 DEFAULT_STORAGE_ENGINE=InnoDB;

CREATE DATABASE db2 DEFAULT_STORAGE_ENGINE=MyISAM;

Almost all system variables that can have both global and session
scope could be set per-database.

Some useful examples:

• AUTOCOMMIT
• DATETIME_FORMAT
• DATE_FORMAT
• DEFAULT_WEEK_FORMAT
• DEFAULT_TMP_STORAGE_ENGINE
• LC_MESSAGES
• LOW_PRIORITY_UPDATES
• SQL_MODE
• TX_ISOLATION

Some global-only variables could also possibly be set per-database:

• FT_STOPWORD_FILE
• FT_QUERY_EXPANSION_LIMIT
• FT_MAX_WORD_LEN
• FT_BOOLEAN_SYNTAX
• FT_MIN_WORD_LEN