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 we are using ```linitial```, we should move this ```Assert``` and make it something like ```Assert(list_length(queryList) == 1```.