Add a feature 'strong cas', developed from 'lease' that mentioned in Facebook's paper.

  I m working on a trading system,  and getting stale data for the  system is unaccepted at most of the time.  But the high throughput make it impossible to get all data from mysql. So i want to make it more reliable  when use memcache as a cache. Facebook's paper "Scaling Memcache at Facebook" mentions a method called ‘lease' and 'mcsqueal', but the mcsqueal is difficult for my case, because it is hard to get the key for mysql.  
   Adding the 'strong cas' feature is devoted to solve the following typical problems,  client A and Client B want to update the same key, and A(set key=>1)update database before B(set key=>2):
key not exist in cache: (A get-miss)->(B get-miss)->(B set key=2) -> (A set key=1); 
or key exist in cache: (A delete key)->(B delete key)->(B set key=2) -> (A set key=1);
Some thing Wrong!  the key=2 in database but key=1 in cache.

It is possible to happen in a high concurrent system, and i don't find a way to solve it with the current cas method. So i add two command 'getss' and 'deletess', they will create a lease and return a cas-unique, or  tell the client there already exist lease on the server. the client can do something to prevent stale data. such as wait, or invalidate the pre-lease.
  I also think the lease is a concept of  'dirty lock', because anybody try to update it will replace itself expiration to the lease's expiration(the lease's expiration time should be very short), so in the worst case(low probability), the stale data only exist in cache for a short time. It is accepted for most app in my case.

  For more detail information, please read doc/strongcas.txt.   And hoping for u guys suggestion ~_~


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	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