autogenerate the memcache_test help output, and pipeline memcache requests

fix doc tpyos, rename some enums, fix printf format len for non-zero terminated strings (hg status), pass args to memcache_cmd_format_header via memcache_req_*, handle zero-length STORE requests, memcache_req is_buf_ours + free, other function name typos (keymemcache_req_key), fix req state behaviour re *_DATA_* for STORE requests and FETCH/END, better memcache_server connpool events/management, modular memcache_test with a working benchmark. This is a long commit message.

add the coro_test.c file, and update .hgignore a bit

bugfixed, enum->string mappings, test does requests

documentation tweaks, add req_abort function prototype

stub functions and documentation

the request/reply code should be complete now, but still needs testing

more reply-receiving code, but still incomplete

sending requests, and partial support for receiving -- incomplete, not tested

*queuing, and add missing file