swim: allow to set codec before cfg

One another problem discovered with UDP broadcast test is that
it can affect other tests, even after termination. Doing
swim:broadcast() on one test a programmer can't be sure, who will
listen it, answer, and break the test scenario.

This commit reduces probability of such a problem by

    * allowance to set a codec before swim:cfg(). It allows to
      protect SWIM nodes of different tests from each other -
      they will not understand messages from other tests. By the
      way, the same problem can appear in real applications too;

    * do not binding again a URI passed by test-run into the
      test and closed here. If a test closes a URI given to it,
      it can't be sure, that next bind() will be successful -
      test-run could already reuse it.

Follow up #3234