BUG#15868 InnoDB thread thrashing

Restoring default innodb_thread_concurrency to 8
This default now matches 5.0

mysql-test/r/innodb.result

@@ -1822,7 +1822,7 @@ Variable_name	Value
 innodb_sync_spin_loops	20
 show variables like "innodb_thread_concurrency";
 Variable_name	Value
-innodb_thread_concurrency	20
+innodb_thread_concurrency	8
 set global innodb_thread_concurrency=1001;
 show variables like "innodb_thread_concurrency";
 Variable_name	Value

sql/mysqld.cc

@@ -5842,7 +5842,7 @@ log and this option does nothing anymore.",
   {"innodb_thread_concurrency", OPT_INNODB_THREAD_CONCURRENCY,
    "Helps in performance tuning in heavily concurrent environments.",
    (gptr*) &srv_thread_concurrency, (gptr*) &srv_thread_concurrency,
-   0, GET_LONG, REQUIRED_ARG, 20, 1, 1000, 0, 1, 0},
+   0, GET_LONG, REQUIRED_ARG, 8, 1, 1000, 0, 1, 0},
   {"innodb_thread_sleep_delay", OPT_INNODB_THREAD_SLEEP_DELAY,
    "Time of innodb thread sleeping before joining InnoDB queue (usec). Value 0"
     " disable a sleep",