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 comments for simplifying the implementation of this function, then we can also make this comment a bit shorter to something like:

```C
/*
 * CStoreProcessUtility is the hook for handling utility commands. This function
 * customizes the behaviour of "COPY cstore_table" and "DROP FOREIGN TABLE
 * cstore_table" commands. For all other utility statements, the function calls
 * the previous utility hook or the standard utility command.
 */
```