Indicates whether an attempt to get the result of a statement should block or fail if the
result is not yet available. Normal SQL commands that can be executed directly on Cloud Spanner
should always have their results available when a sync/flush message is received. COPY
statements do not have that, as they require additional messages after a flush/sync has been
received. Attempts to get the result of a COPY statement should therefore block until it is
available, which is after a CopyDone or CopyFail message has been received.
