not racey
better instrumentation + perf controls

* use go-metrics
expvar doesn't do aggregations we need, so use go-metrics.
also this lets us use a few more types.
we use a few custom patches for go-metrics, hence we currently point to
a fork, but there's PR's for all of them.
in particular, this version of go-metrics syncs its data to expvar,
which can be convenient.

* support sending metrics via carbon proto
* add more metrics, mostly wrt flushing perf

* expose important perf controls
actually, just enforce that an instance key must always be set

seems much saner
also pull redo entries out of buffered conn input
don't unspool at full speed or it drops packets
make conn responsible for its own metrics
clean up spool code

* separate out spooling code from destination
* add instrumentation to know what's going on, to help tuning
spool performance knobs cleanup

better parametrizing of key performance knobs for spool
better values for some params (at least for our env)
clean up no longer used metric
little better file