valuehistogram: new simple, value-based histogram

This change adds a new valuehistogram panel to support value-based
histograms.  Unlike the previous histogram panel which plots the data
using fixed time-interval buckets, this type of histogram buckets the
data according to the values themselves.  This type of graph provides
a high-level perspective of a field's value across the entire time
interval.
histogram: option to enable/disable zero-fill

For some graph types (particularly point style), it's useful to
explicitly avoid filling missing data points with zeros.  For example,
when plotting the min/max/mean value of a particular field, filling with
zeros might present the wrong picture of the data, since the value is
not actually zero but simply missing.