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

diff --git a/Documentation/config-gerrit.txt b/Documentation/config-gerrit.txt
index fb4335b..57694e9 100644
--- a/Documentation/config-gerrit.txt
+++ b/Documentation/config-gerrit.txt
@@ -4152,7 +4152,8 @@
 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::
 +