ExternalIdCache: Use partial loader by default

We've been running the partial loader on googlesource.com for about 10
days now. Compared to the full loader, it is between 10x and 30x faster.

Latency distribution of full reloads:
- p50:  23s
- p90:  57s
- p95:  61s
- p99.9 65s

Latency distribution of partial reloads:
- p50:  0.744s
- p90:  2.8s
- p95:  3.4s
- p99.9 4.0s

We have Gerrit instances with a large number of accounts (for example,
one particular instance has 200.000 accounts). These are included in the
fleet-wide metrics. The latency distribution for parallel reads does
include loading a previously cached state from disk into memory, which
is most likely a driver for the long tail. However, open-source
installations would see this problem only upon startup of the Gerrit
instance, so it's less of a concern. Generally, we can expect numbers on
open-source instances to be even better if all Git data is on local disk
(SSD).

Change-Id: Ia92250c32482d94643cc9aae6264b16571e65c79

Documentation/config-gerrit.txt

@@ -879,7 +879,7 @@ problematic.
 previously cached state. This applies modifications based on the Git
 diff and is almost always faster.
 `cache.external_ids_map.enablePartialReloads` turns this behavior on
-or off. The default is `false`.
+or off. The default is `true`.
 
 cache `"git_tags"`::
 +

java/com/google/gerrit/server/account/externalids/ExternalIdCacheLoader.java

@@ -100,7 +100,7 @@ public class ExternalIdCacheLoader extends CacheLoader<ObjectId, AllExternalIds>
                 .setCumulative()
                 .setUnit(Units.MILLISECONDS));
     this.enablePartialReloads =
-        config.getBoolean("cache", ExternalIdCacheImpl.CACHE_NAME, "enablePartialReloads", false);
+        config.getBoolean("cache", ExternalIdCacheImpl.CACHE_NAME, "enablePartialReloads", true);
   }
 
   @Override