CompactionFilterV2: eliminate an often unnecessary allocation.

If a compaction filter implementation is simply filtering values, then
allocating the "changed values" bitmap is an extra memory allocation
that adds no value. Additionally, the compaction implementation has to
do marginally more work to calculate the offset into the bitmap
(vector<bool> specialization) for each record the filter did not mark
for deletion.

Explicitly handle the case where compact_->value_changed_buf_ is empty.
Merge pull request #384 from msb-at-yahoo/compaction-filter-2-empty-changed-values

CompactionFilterV2: eliminate an often unnecessary allocation.