iterator: Improves indexed iterator interface and implementations
table: indexIter.Get: Returns nil if the value is nil
leveldb: tFilesIter: Fixes that Prev and Next returns true when reaching the end
Fixes some missing iterator Release and SetReleaser implementations; and some missing call to Release
iterator: Improves merged iterator implementations
Copy key slices if needed for extended period
table: blockIter: No need to copy key/value slice anymore