This prevents a function clause like the one seen in basho/riak#469.
Handle tombstones when accumulating sources
this change is pretty much the same idea as #495. +1 merge. Similar to #495 it would be nice for security to move to using the default option to fold/4 instead of the tombstone defined in riak_core_metadata.erl.
I'm waiting for an option to skip tombstones, did not want to change all the folds twice.