Fix BaseApiListingResource when concurrent execution

[awojcicki]

In some scenarios process method doesn't return proper Swagger definition (scan isn't performed)

Sample replication scenario:

• two threads are inside process method
• new empty Swagger instance is created for both on them
• synchronized block is fired for first one producing correct swagger and marks as initialized
• seconds thread starts synchronized block but because it was marked as initialized scan is not performed
• second thread returns incorrect swagger definition

[awojcicki]

@frantuma hey, we identified issue in BaseApiListingResource If you have some time please take a look on this fix. Thank you!