hll: merge should take the maximum, not the sum, of the two registers.

This worked in the test because most of the registers were 0, so sum
ended up being the maximum too.  However, for larger data sets this
would not be the case and the estimates would be off.
count: add Reset()

For creating sliding windows of sketches, it's useful to be able to
clear all the entries to reduce the amount of garbage created so the
sketch can be re-used.