[WIP] Reduce likelihood of fdw prepared statement collisions

Hi All,

I realize using foreign data wrappers with transaction pooling may not be
strictly supported, but for the most part they work. I am however
occasionally noticing errors stemming from the prepared statement names
created by the fdw modify code colliding between sessions/DBs.

Would the development team be open to a patch which somehow makes this less
likely? Something like the attached patch works, but probably isn't ideal?
Perhaps there is a better unique identifier I can use here. I am very new
to the postgres codebase.

Best
*Marco Montagna*