Add support for specifying a query logging level at the SQLKit layer

This patch was authored and released by @gwynne.

Add SQLDatabase.queryLogLevel protocol requirement, defaulting to .debug for databases which don't implement it. Each individual driver must implement support for this logging. This corresponds to the sqlLogLevel configuration in FluentPostgresDriver and is the first step in normalizing support for it at the PostgresKit/MySQLKit/SQLiteKit layer.