added  new ForceReconnectTimeout to write_graphite plugin

This parameter enables a forced close and reopen established connection with graphite backend after the specified Timeout seconds. This behaviour is desired in environments where the connection to the backend of graphite is done through load balancers. Default to 0 seconds ( 0 seconds = disabled ).

This new parameter fixes https://github.com/collectd/collectd/issues/832

with this parameter configured at 21600 seconds ( = 6 hours) all established connections will be rebalanced after any load balancer changes its state.