Improve H2 migration procedure

[The4thLaw]

Description

In order to limit data corruption during H2 updates, Demyo should use the recommended procedure for updates : https://h2database.com/html/tutorial.html#upgrade_backup_restore .

The recommended way to upgrade from one version of the database engine to the next version is to create a backup of the database (in the form of a SQL script) using the old engine, and then execute the SQL script using the new engine.

[The4thLaw]

See h2database/h2database#2078 for instructions on how to open the database in RO first to improve migration.

If you want to know the version of H2's jar, you can use Constants.BUILD_ID, better with reflection for safety, because that class is not a part of API of H2. If you have some opened database, you can use CALL H2VERSION().

If you want to know the create version of a database, you need to open it (possibly in read-only mode for safety), and execute SELECT VALUE FROM INFORMATION_SCHEMA.SETTINGS WHERE NAME = 'CREATE_BUILD'. If version is the same, you can close it and reopen in read-write mode. If it isn't the same, especially if database is newer that you current jar, you need to do something with it. For example, export it to a SQL script, create a new database and fill it with that script.

Unfortunately, there is no way to prevent corruption of a database by IntelliJ if you open it in read-write mode by its database tool, is uses an entirely outdated version of H2. If multiple versions of H2 are in use, you need a standalone H2 Server process and remote corrections to it instead of embedded databases.

[The4thLaw]

The solution is to extends org.h2.tools.Upgrade in order to load the driver from the filesystem rather than Maven Central. We will need a plugin in the POM of demyo-app to download the various H2 versions we used in Demyo and keep all of them with all releases.

[The4thLaw]

Merged to master.