Add key_prefix option to client.
Add key_prefix option to client.

methane/pymemcache,bwalks/pymemcache,duanhongyi/pymemcache,pinterest/pymemcache,sontek/pymemcache,sontek/pymemcache,thedrow/pymemcache,dive-tv/pyelasticache_client,mbrukman/pymemcache,harlowja/pymemcache,ewdurbin/pymemcache,pinterest/pymemcache,adamchainz/pymemcache