MB-38741: [BP] Reduce item pager sleep time from 3600s to 600s

Backport of http://review.couchbase.org/c/kv_engine/+/136285

The frequency at which the expiry pager runs can be set using a
configuration parameter exp_pager_stime. It has a default of 3600s,
which means it is configured to run once an hour.

The default has been unchanged since at least version 2.5.1.

There have been customer issues (see linked CBSEs) where expiry has
impacted front-end operations. Additionally, the ExpiryPager no longer
holds HashTable locks for extended periods (as it originally) did, so
running it more frequently shouldn't be a problem now.

Reducing the interval from 1 hour to 10 mins.

Change-Id: Ia3f3e8dd7fd9c49f3131f9057e48841a3d829385
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/141424
Well-Formed: Build Bot <[email protected]>
Reviewed-by: Dave Rigby <[email protected]>
Tested-by: Dave Rigby <[email protected]>

Author: daverigby

engines/ep/configuration.json

@@ -382,7 +382,7 @@
             "type": "bool"
         },
         "exp_pager_stime": {
-            "default": "3600",
+            "default": "600",
             "descr": "Number of seconds between expiry pager runs.",
             "type": "size_t"
         },

engines/ep/tests/ep_testsuite.cc

@@ -620,7 +620,7 @@ static enum test_result test_expiry_pager_settings(ENGINE_HANDLE *h,
                                                    ENGINE_HANDLE_V1 *h1) {
 
     cb_assert(!get_bool_stat(h, h1, "ep_exp_pager_enabled"));
-    checkeq(3600, get_int_stat(h, h1, "ep_exp_pager_stime"),
+    checkeq(600, get_int_stat(h, h1, "ep_exp_pager_stime"),
             "Expiry pager sleep time not expected");
     set_param(h, h1, protocol_binary_engine_param_flush,
               "exp_pager_stime", "1");