ClassMetadata shouldn't store defaultDb value

Currently if document doesn't have "db" mapping, it is taken from defaultDb configuration and assigned to classmetadata (in ClassMetadataFactory).

But if we have two documentManagers with different defaultDb configuration, second manager's configuration will be ignored, because "db" value is already set in classMetadata (when the 1st manager was created).

It gets worse with metadata caching, as defaultDb setting is only taken in account when metadata is stored in cache.

This affects a common use-case, when each customer has his own database and defaultDb is defined at runtime.