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.
We usually define functions after they are called, and we try to group related functions together.

So, suggestions for function ordering:

1. Move ```CallPreviousProcessUtility``` to right after ```CStoreProcessUtility```,
2. Then, ```CopyCStoreTableStatement```, ```CStoreProcessCopyCommand```, ```CopyIntoCStoreTable```, ```CopyOutCStoreTable```,
3. Then, ```DropppedCStoreFilenameList```, ```DeleteCStoreTableFiles```,
4. Then, ```CStoreTable```.

Please feel free to choose other function orderings that follows our style guide.

If you make changes to the order of function definitions, please also make the corresponding change to the order of function declarations.