A suggestion for a little fix

Hi,

I'm using ElephantDB in combination with S3. When writing data to S3 via the ElephantDBTap I first got the impression that the process is in an endless loop. It constantly logs an info message like:

    INFO s3native.NativeS3FileSystem: Opening '/domains/{your-domain}/domain-spec.yaml' for reading

After investigating this issue with the help of a Java debugger, I've seen that a new DomainStore is instantiated every time `cascading.tap.Tap/hashCode` is invoked (via getIdentifier). The instantiation of the DomainStore causes `org.apache.hadoop.fs.s3native.NativeS3FileSystem` to fetch the domain-spec.yaml file from S3 and to log the above mentioned message.

I don't know, if it is ok to cache the DomainStore in ElephantDBTap? At least it solved the issue, but maybe there is a better way to fix it?

Best regards

Max