Add dedicated Suggest Thread Pool

Add a dedicated suggest thread pool for the suggest API. With the new completion suggest type, which is purely CPU bounded, it makes more sense to have a dedicated thread pool for suggest compared to having it share the search thread pool and "competing" against other search operations.
closes #3698

docs/reference/modules/threadpool.asciidoc

@@ -14,6 +14,10 @@ pools, but the important ones include:
     For count/search operations, defaults to `fixed`,
     size `3x # of available processors`. 
 
+`suggest`::
+    For suggest operations, defaults to `fixed`,
+    size `# of available processors`.
+
 `get`:: 
     For get operations, defaults to `fixed`
     size `# of available processors`.

src/main/java/org/elasticsearch/action/suggest/TransportSuggestAction.java

@@ -73,7 +73,7 @@ public class TransportSuggestAction extends TransportBroadcastOperationAction<Su
 
     @Override
     protected String executor() {
-        return ThreadPool.Names.SEARCH;
+        return ThreadPool.Names.SUGGEST;
     }
 
     @Override

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

@@ -67,6 +67,7 @@ public class ThreadPool extends AbstractComponent {
         public static final String INDEX = "index";
         public static final String BULK = "bulk";
         public static final String SEARCH = "search";
+        public static final String SUGGEST = "suggest";
         public static final String PERCOLATE = "percolate";
         public static final String MANAGEMENT = "management";
         public static final String FLUSH = "flush";
@@ -108,6 +109,7 @@ public class ThreadPool extends AbstractComponent {
                 .put(Names.BULK, settingsBuilder().put("type", "fixed").put("size", availableProcessors).build())
                 .put(Names.GET, settingsBuilder().put("type", "fixed").put("size", availableProcessors).build())
                 .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.FLUSH, settingsBuilder().put("type", "scaling").put("keep_alive", "5m").put("size", halfProcMaxAt5).build())