relay: move gc subscriber creation out of it

GC consumer creation and destroy seemed to only happen in box.cc
with one exception in relay_subscribe(). Lets move it out for
consistency. Now relay can only notify GC consumers, but can't
manage them.

That also makes it harder to misuse the GC by passing some wrong
vclock to it, similar to what was happening in #10047.

In scope of #10047

NO_TEST=refactoring
NO_CHANGELOG=refactoring
NO_DOC=refactoring