Querying keys without Tombstones

[martinsumner]

There is no way presently to query for keys in Riak, with tombstones filtered out. Keylisting, $key queries and find_keys all return keys including tombstones.

There should be an options for at least one of these queries to skip tombstones. This should ease transitioning to delete_mode of keep - having a configurable method of returning key queries without tombs.

[martinsumner]

#8