Improvement on ClientPool
Improvement on ClientPool

 - Added all the commands from Client class (add, replace, etc.)
 - Dynamically create Client objects when needed
 - Allow setting limits on how many Client object will be created and
   cached

Signed-off-by: Mansour <0a138151f0b84cd00a0d46ca219e90c21972910c@oxplot.com>

shipci/torncache-sample,TestingCI/tornado-memcache,vidyar/tornado-memcache