This change fixes the access of unset segments and keys on the reservoir
when the reservoir size is less than the max OR the number of sampled
segments is smaller than the reservoir size. It does so by tucking away
the segments and keys behind methods that return properly sized slices
into the segments/keys arrays.
It also fixes a bug in the housekeeping for the internal index variable
that holds onto how many items in the array have been populated. As part
of this fix, it changes the type of index to int8, which reduces the
size of the reservoir struct by 8 bytes.
The tests have been updated to provide better coverage for this case.
Change-Id: I3ceb17b692fe456fc4c1ca5d67d35c96aeb0a169
93fad70e4b