Implemented COPY <cstore_table> TO ... feature

COPY <cstore_table> TO ... statement is converted to COPY (select * from <cstore_table>) TO ... format and forwarded to native COPY function of postgresql

A minor bug in drop command is fixed to allow dropping of multiple cstore tables at once.
Some suggestions:

1. ```calls previously registered utility hook``` -> ```calls the previously registered utility hook```.
2. ```If there is no utility hook is registered``` -> ```If no utility hook is registered```.
3. ```standard process utility handler``` -> ```the standard process utility handler```.