add and expose preaggregated column familes.
Change NumericSerializer interface to expect class type instead of a column family
Address some PR comments in `NumericSerializer`

* Use the histogram serializer
* Remove stray comment
* Mark unsafe method
Merge branch 'master' into expose_preaggregated_cfs
Remove upper and sum from the percentiles that we store when recording and updating Timer rollups.
keep track of the buffer size we serialize to rather than recompute the size of the rollup.
Merge pull request #148 from gdusbabek/expose_preaggregated_cfs

Expose preaggregated cfs and alter serialization contract