Add debug timers to instant and range queries.

This adds timers around several query-relevant code blocks. For now, the
query timer stats are only logged for queries initiated through the UI.
In other cases (rule evaluations), the stats are simply thrown away.

My hope is that this helps us understand where queries spend time,
especially in cases where they sometimes hang for unusual amounts of
time.
Merge pull request #278 from prometheus/debug/query-timings

Add debug timers to instant and range queries.