swim: sadly remove cache

SWIM sends basically the same message during a round. There was
a microoptimization so as not to reassemble the message on each
step. Now it is getting harder to support that island of
perfectionism, because

    * Soon all the messages will carry all the sections,
      including indirect messages. Their body is smaller, so it
      is not possible to maintain one cached message without
      reducing its maximal size;

    * In big-clusters even without any changes a cached message
      would need to be rebuilt. This is because anti-entropy
      section won't help much unless it is being changed
      frequent enough;

    * In big clusters changes happen often enough to invalidate
      the cached message constantly, unless SWIM would had
      maintained what members are included into the cache, and
      which are not. Then change of a member, not included into
      the message, would not affect the cache. But it would
      complicate the code too much.

Part of #4253