Don'st start streaming after creating a slot in pg_receivexlog

Hi,

Heikki complained about pg_receivexlog --create-slot starting to stream
in his talk in St. Petersburg. Saying that that makes it a hard to
script feature - and I have to agree. Since that option is new to 9.5 we
can should change that behaviour now if we decide to.

Michael, what do you think?

Greetings,

Andres Freund