Use a ocpy on write list for the consumers map to avoid thrash between threads

Once we've finished caching our consumers map the majority of the operations on
this consumersMap list will be traversals inside of the KafkaPoller. Thus
making the performance hit worth it.

What we don't want to happen is to try to iterate through the consumersmap's
list of consumers while receiving new ZK childEvents

Fixes #6

src/main/groovy/com/github/lookout/verspaetung/zk/AbstractTreeWatcher.groovy

@@ -2,6 +2,7 @@ package com.github.lookout.verspaetung.zk
 
 import com.github.lookout.verspaetung.TopicPartition
 
+import java.util.concurrent.CopyOnWriteArrayList
 import groovy.transform.TypeChecked
 
 import org.apache.curator.framework.CuratorFramework
@@ -71,7 +72,7 @@ abstract class AbstractTreeWatcher implements TreeCacheListener {
             this.consumersMap[key] << offset
         }
         else {
-            this.consumersMap[key] = [offset]
+            this.consumersMap[key] = new CopyOnWriteArrayList([offset])
         }
     }