support deletes of columns and rows with buffering

Note:
I was not able to figure out how to get unit tests working in master branch using maven and junit.

However we do run some unit tests in our branch using sbt and scalatest. See here:
https://github.com/tresata/SpyGlass/blob/feat-deletes/src/test/scala/parallelai/spyglass/hbase/SpyGlassSpec.scala