Add failing update tombstones test

Our current delete scheme requires that updates write tombstones for
previous values when the previous value is in an file logically older
than the current write file.  An unrelated change broke that logic, and
now tombstones are written every single time a value is updated.

This test demonstrates this by counting the number of tombstones
expected for repeatedly updating the same key when two data files are
created. Only a tombstone is required, but we get one per update.
Fix extra update tombstones

An unrelated fix in commit 4ab96d4 caused updates replacing a value
in the same file to write extra tombstones that serve no purpose.
Updates are supposed to create a tombstone for previous values only
when the previous value was stored in an older file. Multiple
values for the same key in the same file do not require these extra
tombstones.
Merge pull request #197 from basho/bugfix/extra-tombstones-on-update

Fix extra tombstones on update

Reviewed-by: slfritchie