Stop losing time :/

* There was a problem where a sequence of extremely slow flushes to ceres
caused the clamped timestamp to drift backwards.
* I've decoupled the actual flushing, from the request-to-flush from statsd.
* Hopefully this means in this scenario all that should happen is that the
flushes take a bit longer when catching up, but the time series data won't be lost.
* The clamped timestamps will adjust forward if their deviation from the
system's notion of time is > than the flush interval this means that gaps in data
can appear, but the same timestamp should never get over-written!