Increase 'execution.defaultThreadPoolSize' default and min to 2

GC operation might be time consuming and it prevents the other
tasks to be executed in WorkQueue (e.g. reviewers plugin cannot add
reviewers). Set default and minimum value to 2 so that running GC
(either scheduled or on-demand) is not blocking other background tasks.

Change-Id: I234e8754f235c1e10837dc98b9c4e3b9568676aa
Signed-off-by: Jacek Centkowski <jcentkowski@collab.net>

Documentation/config-gerrit.txt

@@ -4152,7 +4152,8 @@ Default is 256.
 The default size of the background execution thread pool in
 which miscellaneous tasks are handled.
 +
-Default is 1.
+Default and minimum is 2 so that a single, potentially longer executing
+task (e.g. GC), is not blocking the entire execution.
 
 [[execution.fanOutThreadPoolSize]]execution.fanOutThreadPoolSize::
 +

java/com/google/gerrit/server/git/WorkQueue.java

@@ -100,7 +100,7 @@ public class WorkQueue {
 
   @Inject
   WorkQueue(IdGenerator idGenerator, @GerritServerConfig Config cfg, MetricMaker metrics) {
-    this(idGenerator, cfg.getInt("execution", "defaultThreadPoolSize", 1), metrics);
+    this(idGenerator, Math.max(cfg.getInt("execution", "defaultThreadPoolSize", 2), 2), metrics);
   }
 
   /** Constructor to allow binding the WorkQueue more explicitly in a vhost setup. */