KAFKA-4319; Parallelize shutdown of fetchers in AbstractFetcherManager to speedup shutdown

Author: Alexey Ozeritsky <aozeritsky@yandex-team.ru>

Reviewers: Ismael Juma <ismael@juma.me.uk>, Jason Gustafson <jason@confluent.io>

Closes #2023 from resetius/AbstractFetcherManager-shutdown-speedup

core/src/main/scala/kafka/server/AbstractFetcherManager.scala

@@ -118,6 +118,10 @@ abstract class AbstractFetcherManager(protected val name: String, clientId: Stri
 
   def closeAllFetchers() {
     mapLock synchronized {
+      for ( (_, fetcher) <- fetcherThreadMap) {
+        fetcher.initiateShutdown()
+      }
+
       for ( (_, fetcher) <- fetcherThreadMap) {
         fetcher.shutdown()
       }