Only scan parts of the segment which are filled up upon insertion

This was scanning the entire segment (currently 100MB) for every row
insertion, taking ~7s per 1000 rows. This insertion algorithm is n^2 and
probably too slow in general, but at least now the benchmarks (100k
rows) can be run.
Write a basic query parser test
Drop support for "equal", "lessThan" filter names in favor of =, <

The operator version is shorter and less error prone. Equal vs. equals
has tripped me up many times.
Add more error checking to queries, and only allow grouping by dimension cols
Update benchmarks to use metric & dimension columns
Add a method to compute the compression factor

I'm using this during development, but this might be good to expose in a
metricz route or diagnostics route to expose configuration or schema
issues which are preventing compression.
Clean up some comments
Fix a bug where we would only collapse 255 of the same row.
Disable row collapsing until we implement a faster version

It's unworkably slow.