implement socket-alarm plugin

[unbit]

Today a mysql vassal crashed and the customer was not able to detect it promptly.

Instead of adding log alarms for every possibile mysql error situation, implement a plugin doing periodic socket connections and triggering an alarm on error:

[uwsgi]
plugin = socket_alarm
alarm = mysql_is_down xxxx:xxxx
socket-alarm = mysql_is_down 127.0.0.1:3306

[unbit]

Consider integrating it into the api, every container could have a number of SocketMonitor objects mapped to it