txn_limbo: fence upon receiving raft term greater than queue term

Receiving a raft term greater than the current queue term means that
someone has either already written PROMOTE (in case elections are
disabled), or is going to write PROMOTE once he wins the elections (in
case they are enabled).

In both cases the queue owner in an old term should freeze the limbo
until queue term catches up with raft term.

Unfreezing happens automatically once synchro queue term catches up.

Part-of #5295

NO_DOC=covered by next commit