Fine grained Metadata validation

This task is about improve metadata validation to get a finer grained control during Metadata creation.
The current implementation of metadata validate()= method consider only xsd's constraints. IOW it is checking more or less mandatory fields presence and cardinalities of collection. There are much more stuffs that can be verified at a logical level. For example some fields are specifying porperty that make sense only when they are activated by other (boolean) fields, or we can check also related fields for consistency (example: min_pool-size < max_poolsize)