Log SQL Lab query execution phase durations

[mistercrunch]

Now that we have statsD logging setup for DataDog, let's add query phase duration. We'd like to know avg duration time for phases of query with these new keys:

• query_total_duration
• query_pending_duration
• query_execution_duration
• query_results_backend_push_duration

It should be a matter of adding a call to stats_logger.incr(key, value) like here:
https://github.com/airbnb/superset/blob/master/superset/models/core.py#L755

in places like here:
https://github.com/airbnb/superset/blob/master/superset/sql_lab.py#L118