Unset the finalizer when no longer needed, i.e. object explicitly closed/released
iterator: MergedIterator: Check for error may occur after advancing iterator
Tidy up some naked returns
leveldb: version: Remove use of SetFinalizer around internal types

Related to issue #25
iterator: Introduce IteratorSeeker.Release() method

As of right now most Release() method are not yet implemented.

Related to issue #25