Implement database validation

[ghost]

Leverages the unused database attribute (Which is present in the config schema) to constraint the proxy to use the specified database for the target. This is important for security reasons if there are multiple databases and access should only be allowed to specified databases. Also the documentation backs this up: It is assumed that each URL represents the same database (data).

Notes:

• Since the database attribute is not specified in documentation, I don't consider this a breaking change. However, if you are to accept this functionality, I can implement the functionality behind a command line flag etc.
• Works only for the default target. I don't really understand how multiple targets are supported by YAZ proxy. It seems that target node is fetched with proxy target host+port combination using the name attribute of the target but documentation doesn't talk about the format of that attribute.

[ghost]

What about this?