relay: fix PROMOTE and raft term ordering

Fix two issues with sent_raft_term calculations:
* first of all, it doesn't matter during initial and final join, so set it
  to UINT64_MAX.
* secondly, it's nullified after a successful dispatch from the tx
  thread. This might make the relay stall forever. For example, when
  elections are disabled.

NO_DOC=bugfix
NO_TEST=tested in next commit