Add Conflict Resolution in Metadata Folds

using the existing support in iterators
Add Logical Deletion to Cluster Metadata

* similar to ring metadata
* write tombstone values on delete
* handle tombstones on read
* resolvers and modifiers must be tombstone-aware. see typespecs.
* iterators/folds can optionally return the tombstone value or can provide
a default value to return or function to apply when they are encountered
Iterate only a Subset of Cluster Metadata Keys

If keys are tuples then a match-spec like pattern
can be passed as an option to iterator/2 and fold/4 to
filter which keys the iterator visits.