Core: switch to fixed thread pool by default for management threads

Switch management threads to a fixed thread pool with up to 5 threads, and queue size of 100 by default, after which excess incoming requests are rejected.

Closes elastic#7318

Closes elastic#7320

Author: mikemccand

src/main/java/org/elasticsearch/threadpool/ThreadPool.java

@@ -119,7 +119,7 @@ public ThreadPool(Settings settings, @Nullable NodeSettingsService nodeSettingsS
                 .put(Names.SEARCH, settingsBuilder().put("type", "fixed").put("size", availableProcessors * 3).put("queue_size", 1000).build())
                 .put(Names.SUGGEST, settingsBuilder().put("type", "fixed").put("size", availableProcessors).put("queue_size", 1000).build())
                 .put(Names.PERCOLATE, settingsBuilder().put("type", "fixed").put("size", availableProcessors).put("queue_size", 1000).build())
-                .put(Names.MANAGEMENT, settingsBuilder().put("type", "scaling").put("keep_alive", "5m").put("size", 5).build())
+                .put(Names.MANAGEMENT, settingsBuilder().put("type", "fixed").put("size", halfProcMaxAt5).put("queue_size", 100).build())
                 .put(Names.FLUSH, settingsBuilder().put("type", "scaling").put("keep_alive", "5m").put("size", halfProcMaxAt5).build())
                 .put(Names.MERGE, settingsBuilder().put("type", "scaling").put("keep_alive", "5m").put("size", halfProcMaxAt5).build())
                 .put(Names.REFRESH, settingsBuilder().put("type", "scaling").put("keep_alive", "5m").put("size", halfProcMaxAt10).build())