support encoding packet numbers in 3 bytes

Fixes #1424.

quiche/src/packet.rs

diff --git a/quiche/src/packet.rs b/quiche/src/packet.rs
index 3b8273e..39194f0 100644
--- a/quiche/src/packet.rs
+++ b/quiche/src/packet.rs
@@ -560,6 +560,8 @@
         1
     } else if pn < u64::from(u16::MAX) {
         2
+    } else if pn < 16_777_215u64 {
+        3
     } else if pn < u64::from(u32::MAX) {
         4
     } else {