Add interleaving support to tree model.

This commit adds support for interleaved regions and nodes to the tree
model, but does not use it yet. This commit can be tested to verify
that the changes do not impact the non-interleaved case.

src/capture.rs

@@ -1439,15 +1439,25 @@ impl EndpointReader {
 #[derive(Copy, Clone)]
 pub enum CompletionStatus {
     Complete,
-    Ongoing
+    Ongoing,
+    InterleavedComplete(u64),
+    InterleavedOngoing,
 }
 
 impl CompletionStatus {
     pub fn is_complete(&self) -> bool {
         use CompletionStatus::*;
         match self {
-            Complete => true,
-            Ongoing => false,
+            Complete | InterleavedComplete(_) => true,
+            Ongoing | InterleavedOngoing => false,
+        }
+    }
+
+    pub fn is_interleaved(&self) -> bool {
+        use CompletionStatus::*;
+        match self {
+            InterleavedComplete(_) | InterleavedOngoing => true,
+            Complete | Ongoing => false,
         }
     }
 }