Avoid propagating passed in text query directly to worker nodes

@anarazel brought up the following in context of DDL command propagation:

1) The query string can potentially contain multiple SQL statements and such. In that case just sending it to the workers as-is is obviously wrong.

2) The original query depends on the search path and such on the primary, something that's potentially going to be different on workers. We should never just send query text ahead as-is.