Updates buffer overflow handling to occur coincident to other channel

states rather than being a channel state itself.  This was preventing
traffic channels that overflow from being torn down correctly.

src/main/java/io/github/dsheirer/channel/metadata/Attribute.java

@@ -20,6 +20,7 @@ package io.github.dsheirer.channel.metadata;
 
 public enum Attribute
 {
+    BUFFER_OVERFLOW,
     CHANNEL_CONFIGURATION_NAME,
     CHANNEL_CONFIGURATION_SITE,
     CHANNEL_CONFIGURATION_SYSTEM,