Merge branch 'kk/reftable-tombstone-quadratic-fix'

The performance of ref updates and reads using the 'reftable' backend
in the presence of many deletion tombstone records has been optimized
by removing the tombstone suppression flag from the merged iterator
and instead skipping tombstones at higher-level call sites where
iteration bounds are known.

* kk/reftable-tombstone-quadratic-fix:
  reftable: fix quadratic behavior in the presence of tombstones
  t/perf: add perf test for ref tombstone scenarios