Enhance the "Database Mechanism" chapter

[bschmalhofer]

• [x] show an example with bind variables as the first example
• [x] use more interesting examples
• [x] show FetchrowArray() for a single row
• [x] show SelectAll()
• [x] don't mention that incomplete fetches are allowed, as this encourages bad practice
• [x] don't mention direct access to the DBI database handle
• [x] mention MariaDB
• [x] show INSERT and UPDATE with bind variables
• [x] mention that there is only one statement handle per DB object

[bschmalhofer]

TODO:

• [x] mention the magic with _currenttimestamp in the SQL

[bschmalhofer]

More could be said, but closing this issue now.