No support for ssize_t config variables

ep-engine defines many configuration variables with type ssize_t, however trying to change one of these away from the default value does not work as the memcached config parser (accessed via the server API) although defines DT_SSIZE has no case in the processing loop.

Compare (seems all branches are affected)

• No config_parser.cc/c case for DT_SSIZE - http://src.couchbase.org/source/search?q=&defs=&refs=DT_SSIZE&path=&hist=&type=&project=2.5.1&project=3.x&project=sherlock&project=trunk&project=watson
• config_parser.cc/c case for DT_SIZE - http://src.couchbase.org/source/search?q=&defs=&refs=DT_SIZE&path=&hist=&type=&project=2.5.1&project=3.x&project=sherlock&project=trunk&project=watson