MB-47386: Change default max_checkpoints from 2 to 10

All the work in MB-47386 aimed to this change.
Full checkpoint removal is the most effective checkpoint memory
recovery strategy. Increasing the number of allowed checkpoints leads
to having much more checkpoints eligible for removal.

Change-Id: Iccf03688b143ae6516f317536ba99a1d60f18dfb
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/163334
Tested-by: Build Bot <[email protected]>
Reviewed-by: Dave Rigby <[email protected]>

Author: paolococchi

engines/ep/configuration.json

@@ -752,7 +752,7 @@
             }
         },
         "max_checkpoints": {
-            "default": "2",
+            "default": "10",
             "descr": "Max number of checkpoints in each VBucket.",
             "dynamic": true,
             "type": "size_t",

engines/ep/tests/module_tests/checkpoint_remover_test.cc

@@ -27,7 +27,13 @@ void CheckpointRemoverTest::SetUp() {
     if (!config_string.empty()) {
         config_string += ";";
     }
-    config_string += "max_vbuckets=8;checkpoint_remover_task_count=2";
+    // Note: By MB-47386 the default max_checkpoints has be set to 10 but we
+    // still run this testsuite with the original max_checkpoints=2. Reason is
+    // that a bunch of (logic) tests inspect the inner checkpoint queues and
+    // rely on max_checkpoints=2 for knowing where to find items. Note that
+    // max_checkpoints doesn't have any logic-change implication.
+    config_string +=
+            "max_vbuckets=8;checkpoint_remover_task_count=2;max_checkpoints=2";
 
     SingleThreadedKVBucketTest::SetUp();
 }