Set sshd.threads to at least 4.

Otherwise, weird hangs can occur, for example when using Git LFS.

See https://github.com/git-lfs/git-lfs/issues/3727

Change-Id: Ic19cb136bb37d62d8339b85c514c180f733e3465

Documentation/config-gerrit.txt

@@ -4239,7 +4239,8 @@ Number of threads to use when executing SSH command requests.
 If additional requests are received while all threads are busy they
 are queued and serviced in a first-come-first-served order.
 +
-By default, 2x the number of CPUs available to the JVM.
+By default, 2x the number of CPUs available to the JVM (but at least 4
+threads).
 +
 [NOTE]
 When SSH daemon is enabled then this setting also defines the max number of

gerrit-server/src/main/java/com/google/gerrit/server/config/ThreadSettingsConfig.java

@@ -28,7 +28,7 @@ public class ThreadSettingsConfig {
   @Inject
   ThreadSettingsConfig(@GerritServerConfig Config cfg) {
     int cores = Runtime.getRuntime().availableProcessors();
-    sshdThreads = cfg.getInt("sshd", "threads", 2 * cores);
+    sshdThreads = cfg.getInt("sshd", "threads", Math.max(4, 2 * cores));
     httpdMaxThreads = cfg.getInt("httpd", "maxThreads", 25);
     int defaultDatabasePoolLimit = sshdThreads + httpdMaxThreads + 2;
     databasePoolLimit = cfg.getInt("database", "poolLimit", defaultDatabasePoolLimit);