Now that much of walreceiver has been pulled back into the postgres

Now that much of walreceiver has been pulled back into the postgres
binary, revert PGDLLIMPORT decoration of global variables. I'm not sure
if there's any real harm from unnecessary PGDLLIMPORTs, but these are all
internal variables that external modules really shouldn't be messing
with. ThisTimeLineID still needs PGDLLIMPORT.