message: In TLA+ spec, Raft leader should only advance its commit index in current term
diff: @@ -550,7 +550,10 @@ AdvanceCommitIndex(i) ==
  /\ state[i] = Leader
  /\ LET
  \* We want to get the smallest such index forward that is a signature
  - new_index == SelectInSubSeq(log[i], commitIndex[i]+1, Len(log[i]), HasTypeSignature)
  + \* This index must be from the current term,
  + \* as explained by Figure 8 and Section 5.4.2 of https://raft.github.io/raft.pdf
  + new_index == SelectInSubSeq(log[i], commitIndex[i]+1, Len(log[i]),
  + LAMBDA e : e.contentType = TypeSignature /\ e.term = currentTerm[i])
  new_log ==
  IF new_index > 1 THEN
  [ j \in 1..new_index |-> log[i][j] ]
  