Add new functions 'uwsgi_metric_set_min' & 'uwsgi_metric_set_max', which...

push respectively a absolute value if she is less than or greater than
that the previous push.

Refactore the function uwsgi_metric_set_min
Exposure of 'uwsgi_metric_set_min' & 'uwsgi_metric_set_max' for python interface.
Doesn't use uwsgi_metric_get for the comparison on uwsgi_metric_set_max and uwsgi_metric_set_min (it introduce a deadlock)
Merge pull request #631 from btall/set-min-set-max

Add new functions 'uwsgi_metric_set_min' & 'uwsgi_metric_set_max', wh…