Add a way to configure statistics, turn on statistics gathering 20% of
the time, don't dump the statistics if we didn't configure them.
Fix a bug where the test for a configured RLE was checking for an
explicit value of 0 instead of "< 2": variable length column-store
values, that normally pass in an RLE of 1, could set a value cell's
length to a negative number, potentially causing overflow in the
dictionary-match function.

Rewrite the dictionary match function so values with no RLE can match
values with an RLE.

Close #661.