Revert "zyre_new uses its address instead of generating random ids."

This reverts commit d3335b2c8586438d396831017d6ba6caa5c7187b.

Problem: while this commit seemed logical and clean, it introduced a
random failure, when creating, destroying, and recreating zyre
objects. That is, they would bind to the same inproc endpoint, as
the same pointer address was reallocated by the heap. The result is
random failures in "make check" and other applications.

Solution: switch back to the old random/loop approach.
Merge pull request #274 from hintjens/master

Revert "zyre_new uses its address instead of generating random ids."