fix to handle max open file limit
add curr_connections and total_connections counter and put into stats
maintain a safe number for max client connections to solve ENFILE issue
add frag_owner counter
After rebase: mget split to many mget and send to backend

reabse following::

    * 0fd32bb - (HEAD, idning/mget-improve, mget-improve) unchange (10 hours ago) <idning>
    * 705acab - clean code (10 hours ago) <idning>
    * 32e8559 - correct, but not clean. (18 hours ago) <idning>
    * fde1f12 - mget-improve almost done. (2 days ago) <idning>
    * b30fa4f - mget split to many mget and send to backend (3 days ago) <idning>
    * 7cf5625 - add frag_owner counter (4 days ago) <idning>

    * 8a4f5c0 - (origin/master, origin/HEAD, idning/master) Update README.md (12 days ago) <Manju Rajashekhar>
fix multi DEL, a bug on parse key, and remove assert on conn_sendv
add benchmark code
MSET/MSETNX support now
follow the coding style
enhance error handling on backend down
fix bug on msg_fragment_argx when key_len=mbuf_size-1
1. get/gets for memcache passed cases
2. fix req_done(nfrag_done) for memcache
1. do not rewrite the orig mget as a ping msg for redis, and del useless code
2. nfrag_done work for memcache
MSETNX should not support

MSETNX Return:

- 1 if the all the keys were set.
- 0 if no key was set (at least one key already existed).
adjust style
add msg->fragement handler, main logic of mget-improve is put into proto/
put hash_tag logic into server_pool_idx
reduce mem usage in fragement
formating
remove dead code

remove:
- msg_fragment
- MSG_PARSE_FRAGMENT:
- first_fragment/last_fragment