This patch removes most remaining delays due to uninteruptible sleep()
during shutdown, as found using PMP. This makes standard test run very
close in speed to with --loose-innodb-fast-shutdown=2, and greatly
speeds up running the test suite.

sql/mysqld.cc:
  Poll for threads to die every 20 msec during shutdown, rather than force
  a wait for 2 full seconds should one thread be a little slow to exit
  (this was seen occasionally in mysql-test-run).
storage/xtradb/include/srv0srv.h:
  Rename the event, as we now use it to wakeup more threads during shutdown.
storage/xtradb/log/log0log.c:
  Rename the event, as we now use it to wakeup more threads during shutdown.
storage/xtradb/srv/srv0srv.c:
  Replace some hardcoded sleep()s with os_wait_event_time() that can be
  interrupted early during server shutdown to avoid unnecessary delays.