[finagle-memcached] apply offloading of responses after gathering response parts

# Problem
Finagle offloads responses from clients ASAP to free up netty threads.
If a multikey request to memcached is partitioned across memcached endpoints, each response of such subrequest is offloaded.
This means additional inter-thread synchonisation and even additional delay while subresponses are waiting in the offload queue.

# Solution
Offload responses after all subresponses are collected.

JIRA Issues: STOR-8861

Differential Revision: https://phabricator.twitter.biz/D1184723

Author: Anton Ivanov

finagle-memcached/src/main/scala/com/twitter/finagle/Memcached.scala

@@ -6,6 +6,7 @@ import com.twitter.hashing
 import com.twitter.finagle.client._
 import com.twitter.finagle.dispatch.SerialServerDispatcher
 import com.twitter.finagle.dispatch.StalledPipelineTimeout
+import com.twitter.finagle.filter.OffloadFilter
 import com.twitter.finagle.liveness.FailureAccrualFactory
 import com.twitter.finagle.liveness.FailureAccrualPolicy
 import com.twitter.finagle.loadbalancer.Balancers
@@ -268,6 +269,12 @@ object Memcached extends finagle.Client[Command, Response] with finagle.Server[C
               BindingFactory.role,
               MemcachedPartitioningService.module
             )
+            // We want offloading to happen after partitioning, i.e. after all responses are collected
+            // to reduce pressure on offload pool
+            .remove(OffloadFilter.Role)
+            .insertBefore(
+              MemcachedPartitioningService.role,
+              OffloadFilter.client[Command, Response])
             // We want this to go after the MemcachedPartitioningService so that we can get individual
             // spans for fanout requests. It's currently at protoTracing, so we remove it to re-add below
             .remove(MemcachedTracingFilter.memcachedTracingModule.role)