We should do some ITs for ConcurrentBufferedMutator.

• creation
• performing a lot of mutations, closing the connection and verifying integrity of databases
• errors thrown to the user when primary is throwing errors (mutates before flush shouldn't throw any errors, only explicit flush of a mutate that happens after async flush is performed (I think that the former case will be easier to IT, the latter should be tested in a unit test - we should verify if it is true). Thrown exception should be RetiresExhausted... exception with MirroringOperationException.extractRootCause(e).databaseIdentifier == Primary and correct mutation stored in .operation field.
• The same case for failing secondary, but with databaseIdentifier == Secondary.
• We are not able to perform such a tests for Both databases throwing an exception now, it is tested by unit tests.
  Be aware that ConcurrentBufferedMutator shouldn't report errors to the faillog.