Classify certain errors as retryable

Impala should be able to classify certain errors as "retryable". This can be done by modifying the TStatus object to have a "type". For now, the only types can be "GENERAL" and "RETRYABLE". This way when a TStatus object is created, it can be marked as retryable. If the TStatus is retryable, the Coordinator can trigger a retry of the query.

This approach allows us to incrementally mark more and more errors as retryable as necessary. For now, just RPC failures will be marked as retryable.