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.
If you agree with my suggestions for ```CStoreProcessUtility```, then we can simplify the implementation of this function, since we already know that the relation is a cstore table.