Unify schema version number and schema file name in output

[oxzi]

An Icinga DB user complained that the error message for a missed schema update is not so helpful, since it prints the schema versions instead of the Icinga DB versions, but even the schema updates are named after the Icinga DB version, not the internally used integer. A map translating this might be useful.

Originally posted by @oxzi in #789 (comment)

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Instead of introducing an internal map between both the MySQL/MariaDB and PostgreSQL schema version to the file names, another column could be introduced to the icingadb_schema table.