add notice-log for every request

we need a **NOTICE** log for every request, fields::

    type, key, req_len, rsp_len, request_time (like access_log for nginx) 

so we can stat req/s, average srv time, key frequency with awk/grep, this is very useful for Tracing.

here is the format:

    [Fri May 30 16:43:03 2014] nc_request.c:102 notice req 3 done on c 11 req_time: 0.265 type: SET narg: 3 req_len: 35 rsp_len: 5 key0: kkk-3, done: 1, error:0
    [Fri May 30 16:43:03 2014] nc_request.c:102 notice req 23 done on c 11 req_time: 0.159 type: MGET narg: 11 req_len: 11 rsp_len: 16 key0: kkk-3, done: 1, error:0
    [Fri May 30 16:43:03 2014] nc_request.c:102 notice req 43 done on c 11 req_time: 0.121 type: DEL narg: 11 req_len: 11 rsp_len: 5 key0: kkk-3, done: 1, error:0
    [Fri May 30 16:43:03 2014] nc_request.c:102 notice req 63 done on c 11 req_time: 0.108 type: MGET narg: 11 req_len: 11 rsp_len: 10 key0: kkk-3, done: 1, error:0
maybe add:

- client/backend addr
- log timestamp should ms/us.  [Wed May 28 15:06:12.xxx 2014]

space between the closing paren and curly brace. So log_loggable(LOG_NOTICE) {
Here are the coding guidelines -- https://github.com/twitter/twemproxy/blob/master/notes/c-styleguide.txt. Would really appreciate if you follow it