Allow persistent caches to be not persisted by default

H2CacheFactory already respected a non-positive value and disabled
persistence when explicitly configured in gerrit.config. However, the
implementation of PersistentCacheProvider would never return a
non-positive value from diskLimit(), so it was impossible to disable
persistence by default. Fix this so PersistentCacheProvider uses 128 MiB
when the instance is created, but allow overriding with a non-positive
value.

Change-Id: I189f6dad2003c8bb8ba6e05705e2903c099759c3

Documentation/config-gerrit.txt

@@ -776,7 +776,7 @@ Default is 128 MiB per cache, except:
 * `"diff_summary"`: default is `1g` (1 GiB of disk space)
 
 +
-If 0, disk storage for the cache is disabled.
+If 0 or negative, disk storage for the cache is disabled.
 
 ==== [[cache_names]]Standard Caches
 

java/com/google/gerrit/server/cache/PersistentCacheBinding.java

@@ -34,7 +34,12 @@ public interface PersistentCacheBinding<K, V> extends CacheBinding<K, V> {
 
   PersistentCacheBinding<K, V> version(int version);
 
-  /** Set the total on-disk limit of the cache */
+  /**
+   * Set the total on-disk limit of the cache.
+   *
+   * <p>If 0 or negative, persistence for the cache is disabled by default, but may still be
+   * overridden in the config.
+   */
   PersistentCacheBinding<K, V> diskLimit(long limit);
 
   PersistentCacheBinding<K, V> keySerializer(CacheSerializer<K> keySerializer);

java/com/google/gerrit/server/cache/PersistentCacheProvider.java

@@ -39,6 +39,7 @@ class PersistentCacheProvider<K, V> extends CacheProvider<K, V>
       CacheModule module, String name, TypeLiteral<K> keyType, TypeLiteral<V> valType) {
     super(module, name, keyType, valType);
     version = -1;
+    diskLimit = 128 << 20;
   }
 
   @Inject(optional = true)
@@ -93,10 +94,7 @@ class PersistentCacheProvider<K, V> extends CacheProvider<K, V>
 
   @Override
   public long diskLimit() {
-    if (diskLimit > 0) {
-      return diskLimit;
-    }
-    return 128 << 20;
+    return diskLimit;
   }
 
   @Override