Fix MultiLevelTimeseries::getRate()

Fix MultiLevelTimeseries::getRate()

Summary:I hit a landmine where rate() returned 0 for my tests where fewer than 60 items were added per minute.  This was because it was truncating it.  And yet, countRate() was working.
It doesn't make sense for a rate (value accumulated per time period) to be integral.
Folly: I changed rate() to return a double by default for folly, as was done by avg() and countRate().  Looked like a typo to me.
Common wrapper: I changed getRate() to allow you to override and make it double, as was done by getAvg().  Defaulting to int (which is usually the value type) is a bad call IMO but it's a riskier change to change it to double, and I want to be consistent with getAvg().

Reviewed By: tracelog

Differential Revision: D2921061

fb-gh-sync-id: 00875f2ab7963ef3ba2db475aedaf6ebd413b38f
shipit-source-id: 00875f2ab7963ef3ba2db475aedaf6ebd413b38f