Parameters to SQL functions are not supported

As SQL function contents are always executed with prepared statement parameters in place, without using the plancache/custom plan logic, our current implementation of prepared statements doesn't work for SQL functions.

The current state of prepared statement support is:

| | prepare | pl/pgsql | sql function |
| :-- | :-- | :-- | :-- |
| **Router Executor** | | | |
| Select - Partition Column | Always | Always | Fails (uses real-time) |
| Insert - Partition Column | Always | Always | Fails |
| Update - Partition Column | Always | Always | Fails |
| Delete - Partition Column | Always | Always | Fails |
| Select - Non-Partition Column | Always | Always | Fails (uses real-time) |
| Insert - Non-Partition Column | Always | Always | Always |
| Update - Non-Partition Column | Always | Always | Always |
| Delete - Non-Partition Column | Always | Always | Always |
| **Real-time Executor** | | | |
| Partition Column | Always | Always | Fails |
| Non-Partition Column | Always | Always | Fails |
| **Task-tracker Executor** | | | |
| Partition Column | Always | Always | Fails |
| Non-Partition Column | Always | Always | Fails |

There are no immediate plans to implement parameters to SQL functions. Therefore for now the fix is to use plpgsql (or other languages) instead of sql functions.