Previously, COPY use CdbDispatchUtilityStatement directly to
dispatch 'COPY' statements to all QEs and then send/receive
data from primaryWriterGang, this way happens to work because
primaryWriterGang is not recycled when a dispatcher state is
destroyed. This seems nasty because the COPY command has finished
logically.

This commit splits the COPY dispatching logic to two parts to
make it more reasonable.