SONAR-12686 MaxDirectMemorySize JVM option should be defined for ES

This option was set by default in ES 6.X already but we had missed it.
ES sets it to half the max heap from Java code (see class JvmErgonomics)

To not have to get into the complexity of interpreting options to know what is the configured max heap,
we just add this option to the defaults and document it

server/sonar-process/src/main/java/org/sonar/process/ProcessProperties.java

@@ -75,7 +75,7 @@ public class ProcessProperties {
     SEARCH_HOST("sonar.search.host", InetAddress.getLoopbackAddress().getHostAddress()),
     SEARCH_PORT("sonar.search.port", "9001"),
     SEARCH_HTTP_PORT("sonar.search.httpPort"),
-    SEARCH_JAVA_OPTS("sonar.search.javaOpts", "-Xmx512m -Xms512m -XX:+HeapDumpOnOutOfMemoryError"),
+    SEARCH_JAVA_OPTS("sonar.search.javaOpts", "-Xmx512m -Xms512m -XX:MaxDirectMemorySize=256m -XX:+HeapDumpOnOutOfMemoryError"),
     SEARCH_JAVA_ADDITIONAL_OPTS("sonar.search.javaAdditionalOpts", ""),
     SEARCH_REPLICAS("sonar.search.replicas"),
     SEARCH_MINIMUM_MASTER_NODES("sonar.search.minimumMasterNodes"),

sonar-application/build.gradle

@@ -97,7 +97,7 @@ task zip(type: Zip, dependsOn: [configurations.compile]) {
     from file('src/main/assembly/conf/sonar.properties')
       filter(ReplaceTokens, tokens: [
               'searchDefaultHeapSize': '512MB',
-              'searchJavaOpts'       : '-Xmx512m -Xms512m -XX:+HeapDumpOnOutOfMemoryError',
+              'searchJavaOpts'       : '-Xmx512m -Xms512m -XX:MaxDirectMemorySize=256m -XX:+HeapDumpOnOutOfMemoryError',
               'ceDefaultHeapSize'    : '512MB',
               'ceJavaOpts'           : '-Xmx512m -Xms128m -XX:+HeapDumpOnOutOfMemoryError',
               'webDefaultHeapSize'   : '512MB',

sonar-application/src/main/assembly/conf/sonar.properties

@@ -251,7 +251,8 @@
 #--------------------------------------------------------------------------------------------------
 # ELASTICSEARCH
 # Elasticsearch is used to facilitate fast and accurate information retrieval.
-# It is executed in a dedicated Java process. Default heap size is @searchDefaultHeapSize@.
+# It is executed in a dedicated Java process. Default maximum heap size is @searchDefaultHeapSize@.
+# It is recommended to also set MaxDirectMemorySize (-XX:MaxDirectMemorySize) and set it to half the maximum heap size.
 #
 # --------------------------------------------------
 # Word of caution for Linux users on 64bits systems