Add a feature 'strong cas':
Strong cas:
---------------------
    It is devoted to prevent setting stale data to the cache at multi-clients
environment, and also prevent thundering herds phenomenon.
----------------------
  For more detail information, please read doc/strongcas.txt. ~_~

	new file:   doc/strongcas.txt
	modified:   items.c
	modified:   memcached.c
	modified:   memcached.h
	modified:   slabs.c
	modified:   t/binary.t
	modified:   t/lib/MemcachedTest.pm
	modified:   t/stats.t
	new file:   t/strongcas.t
	modified:   thread.c