Use consistent quoting for #define'd SQL commands

We don't have a consistent quoting policy for #define'd SQL commands in CItusDB. For example, we have the following declarations in include/distributed/worker_protocol.h:

```
#define TRANSMIT_REGULAR_COMMAND "COPY \"%s\" TO STDOUT WITH (format 'transmit')"
#define COPY_OUT_COMMAND "COPY %s TO STDOUT"
#define COPY_IN_COMMAND "COPY %s FROM '%s'"
```

Andres notes that we should use double quoting everywhere (is this all table names or does it also include filenames?) - right now it'd e.g. would be impossible to use the other commands on a table that starts with numbers or have spaces. To be really correct we should do proper quoting.