WL#8316: Enabled multiple page cleaners and purge threads by default in 5.7

We make the following compiled-in defaults changes in 5.7GA:
innodb_page_cleaners=4
innodb_purge_threads=4

We would like to do a round of QA and SysQA tests to ensure that this causes no
stability or performance issues. 

FR1: Set default innodb_page_cleaners from 1 to 4. But the actual
innodb_page_cleaners would be affected by innodb_buffer_pool_instances as well.
If the number of innodb_page_cleaners is bigger than
innodb_buffer_pool_instances, server would set it to the exact the number of
innodb_buffer_pool_instances.

FR2: Set default innodb_purge_threads from 1 to 4.

NFR1: There should not be any stability or performance issues.

We need to change the default values in these places:
 
 static MYSQL_SYSVAR_ULONG(purge_threads, srv_n_purge_threads,
   PLUGIN_VAR_OPCMDARG | PLUGIN_VAR_READONLY,
-  "Purge threads can be from 1 to 32. Default is 1.",
+  "Purge threads can be from 1 to 32. Default is 4.",
   NULL, NULL,
-  1,			/* Default setting */
+  4,			/* Default setting */
   1,			/* Minimum value */
   32, 0);		/* Maximum value */
 
 static MYSQL_SYSVAR_ULONG(page_cleaners, srv_n_page_cleaners,
   PLUGIN_VAR_OPCMDARG | PLUGIN_VAR_READONLY,
-  "Page cleaner threads can be from 1 to 64. Default is 1.",
-  NULL, NULL, 1, 1, 64, 0);
+  "Page cleaner threads can be from 1 to 64. Default is 4.",
+  NULL, NULL, 4, 1, 64, 0);
 
 /* The number of page cleaner threads to use.*/
-ulong	srv_n_page_cleaners = 1;
+ulong	srv_n_page_cleaners = 4;
  
 /* The number of purge threads to use.*/
-ulong	srv_n_purge_threads = 1;
+ulong	srv_n_purge_threads = 4;

Apart from this, we don't need to change others.