ssize_t ==> lcb_ssize_t

Change-Id: Ibe8ebc44b99ed83d7097429d28c54c5d6df7629b
Reviewed-on: http://review.couchbase.org/25287
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>