web: add more stats to front page

[efahl]

@aparcar
Here's what I've got so far.

Reworked the internal stats structures so that there's now a summary data collection, which makes the code in asu/routers/stats.py a lot cleaner.

Summary stats key prefix is stats:build:*
- requests - total number of calls to the /build API
- cache-hits - count of build requests satisfied by already-existing builds
- cache-misses - count of build requests sent to builder
- successes - count of build requests sent to builder with successful completion
- failures - count of build requests sent to builder that failed

where cache-misses = successes + failures, except for requests that fail validation (which I'm guessing is extremely rare).

I've written a little utility script to convert some of the old data to the new format, not sure if it should be automatic or not... In any case, some of the old stuff is completely broken, like old stats:cache-hits was being added all wrong, so the current values are useless.

Here's what the graph looks like, is this sort of what you were thinking?

[efahl]

I did some cleanup on the time vector, the aggregation buckets are now aligned to midnight, hence the "tomorrow" tag in the hover legend above. I'm thinking of just dropping the time value from the label since it's always 00:00:00Z, and will certainly be shifting back a day, so that the last point is always today.