prepared statements: explicit support for EXEC SQL DEALLOCATE PREPARE name

[GitMensch]

The PG documentation and others always specify "When you don't need the prepared statement anymore, you should deallocate it".

Trying it wiuth ocesql raises an error about unknown prepared statements - the reason behind this is that all prepared statements in ocesql are not handled by the DB, but in the DB lib (for whatever historic reason).

Still the prepared statements need memory, at least the prepare_list entry.

So to both circumvent strange errors from the DB about prepared statements not being known and to prevent resource leak I suggest to add EXEC SQL DEALLOCATE PREPARE to the parser and create a matching OCESQLPrepareDeallocate which will be called and free resources + drop the entry out of the internal list.