Enable sshd.batchThreads by default

Change default value of sshd.batchThreads from 0 to 1 or 2 depending on
the core counts. When Gerrit runs on single code node 1 will be used as
default, 2 otherwise.

Change-Id: I4c04157e9edead9cdf0105aebc5d1658fa60f494
Signed-off-by: Dariusz Luksza <dariusz@luksza.org>

Documentation/config-gerrit.txt

@@ -3427,7 +3427,7 @@ If the number of threads requested for non-interactive users is larger
 than the total number of threads allocated in sshd.threads, then the
 value of sshd.threads is increased to accommodate the requested value.
 +
-By default, 0.
+By default is 1 on single core node, 2 otherwise.
 
 [[sshd.streamThreads]]sshd.streamThreads::
 +

gerrit-sshd/src/main/java/com/google/gerrit/sshd/CommandExecutorQueueProvider.java

@@ -35,7 +35,7 @@ public class CommandExecutorQueueProvider implements QueueProvider {
       final WorkQueue queues) {
     final int cores = Runtime.getRuntime().availableProcessors();
     poolSize = config.getInt("sshd", "threads", 3 * cores / 2);
-    batchThreads = config.getInt("sshd", "batchThreads", 0);
+    batchThreads = config.getInt("sshd", "batchThreads", cores == 1 ? 1 : 2);
     if (batchThreads > poolSize) {
       poolSize += batchThreads;
     }