Safe max connections to handle reaching max open file limit

Issue: https://github.com/twitter/twemproxy/issues/97

I'm trying to fix the 'reaching max open file limit'.

At first, I just ignore the EMFILE/ENFILE and setting `recv_ready` to 0.
But, if there are more pending events, setting `recv_ready` to 0 will lost these events. Sometimes the twemproxy can go back to normal state. 
This fix is just a little better than closing the proxy socket directly.

Then, I found the way redis handling max connection is graceful. 
Here we go. The new fix maintains a safe max number for client connections and checking the current connections after every accept. So it won't get EMFILE or ENFILE theoretically.

We're starting to use the new version in production.

BTW, it adds new stats `curr_connections` and `total_connections`.

Rename
total_connections to ntotal_conn
curr_connections to nconn
curr_client_connections to nclient_conn

curr_connections should be int32_t
curr_client_connections should be int32_t