Fix race condition between table creation and namespace deletion

[DomGarguilo]

Fixes #5389

• fixes a race condition where NamespaceOperationsImpl.delete() checked if a namespace was empty and then submitted the delete FATE op. If another client tried to create a table in that namespace in between the namespace emptiness check and when the DeleteNamespace was actually executed, the table could still be created and also the namespace deleted.
• moves the check into the DeleteNamespace.call() itself which happens under a write lock and the race can't happen anymore.
• added a new NAMESPACE_NOTEMPTY thrift enum so the manager now throws AcceptableThriftTableOperationException with that type which the client catches and converts to NamespaceNotEmptyException
• added a test to mock the DeleteNamespace call on a non-empty namespace to verify the new exception is thrown