remove writeOpQueueFactory

for writeQueueFactory the buffer of the element is nulled by memcache
before remove is called, thus we are not able to determine the size of
element while removing, We can potentially work around this by adding a
wrapper to keep track of size but that will have its extra overhead,
Also verified from the heap dump that the growing queue was the queue
for opQueue.

server/src/main/java/io/druid/client/cache/MemcachedCache.java

@@ -69,7 +69,6 @@ public class MemcachedCache implements Cache
                                         .setOpQueueMaxBlockTime(config.getTimeout())
                                         .setOpTimeout(config.getTimeout())
                                         .setOpQueueFactory(queueFactory)
-                                        .setWriteOpQueueFactory(queueFactory)
                                         .build(),
           AddrUtil.getAddresses(config.getHosts())
         ),