[Consensus] use Range internally for CommitRange (MystenLabs#18117)

## Description 

Maintain backward compatibility with the initial implementation of
CommitRange.

## Test plan 

CI

---

## Release notes

Check each box that your changes affect. If none of the boxes relate to
your changes, release notes aren't required.

For each box you select, include information after the relevant heading
that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK:

consensus/core/src/commit.rs

@@ -3,9 +3,9 @@
 
 use std::{
     cmp::Ordering,
-    fmt::{self, Display, Formatter},
+    fmt::{self, Debug, Display, Formatter},
     hash::{Hash, Hasher},
-    ops::{Deref, RangeInclusive},
+    ops::{Deref, Range, RangeInclusive},
     sync::Arc,
 };
 
@@ -547,32 +547,37 @@ impl CommitInfo {
 
 /// CommitRange stores a range of CommitIndex. The range contains the start (inclusive)
 /// and end (inclusive) commit indices and can be ordered for use as the key of a table.
-#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
-pub(crate) struct CommitRange(RangeInclusive<CommitIndex>);
+///
+/// NOTE: using Range<CommitIndex> for internal representation for backward compatibility.
+/// The external semantics of CommitRange is closer to RangeInclusive<CommitIndex>.
+#[derive(Clone, Default, PartialEq, Eq, Serialize, Deserialize)]
+pub(crate) struct CommitRange(Range<CommitIndex>);
 
 impl CommitRange {
     pub(crate) fn new(range: RangeInclusive<CommitIndex>) -> Self {
-        Self(range)
+        // When end is CommitIndex::MAX, the range can be considered as unbounded
+        // so it is ok to saturate at the end.
+        Self(*range.start()..(*range.end()).saturating_add(1))
     }
 
     // Inclusive
     pub(crate) fn start(&self) -> CommitIndex {
-        *self.0.start()
+        self.0.start
     }
 
-    // Exclusive
+    // Inclusive
     pub(crate) fn end(&self) -> CommitIndex {
-        *self.0.end()
+        self.0.end.saturating_sub(1)
     }
 
     /// Check whether the two ranges have the same size.
     pub(crate) fn is_equal_size(&self, other: &Self) -> bool {
-        self.0.size_hint() == other.0.size_hint()
+        self.0.end.wrapping_sub(self.0.start) == other.0.end.wrapping_sub(other.0.start)
     }
 
     /// Check if the provided range is sequentially after this range.
     pub(crate) fn is_next_range(&self, other: &Self) -> bool {
-        &self.end() + 1 == other.start()
+        self.0.end == other.0.start
     }
 }
 
@@ -592,13 +597,14 @@ impl PartialOrd for CommitRange {
 
 impl From<RangeInclusive<CommitIndex>> for CommitRange {
     fn from(range: RangeInclusive<CommitIndex>) -> Self {
-        Self(range)
+        Self::new(range)
     }
 }
 
-impl Default for CommitRange {
-    fn default() -> Self {
-        Self(CommitIndex::default()..=CommitIndex::default())
+/// Display CommitRange as an inclusive range.
+impl Debug for CommitRange {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        write!(f, "CommitRange({}..={})", self.start(), self.end())
     }
 }