Added the OPT_SERVER_TIMEOUT_LIMIT behaviour.

The option MEMCACHED_BEHAVIOR_SERVER_TIMEOUT_LIMIT was added in libmemcached 1.0.18 (0x01000018).

For more information see https://answers.launchpad.net/libmemcached/+question/239497

The merge request for libmemcached:
https://code.launchpad.net/~493pocbrcycmdw7yksonho9o2qzz-o18bz-d18ecat4t1b76tkfi3vttrkfngli/libmemcached/feature-server_timeout