Cleanup ResultSourceHandle handling after M.A.D. introduction

Cleanup ResultSourceHandle handling after M.A.D. introduction

Since a source/schema combos no longer leak, it is safe to store
strong-refs to sources directly in Row/ResultSet objects. Reduce
ResultSourceHandle to a simple Source "meta-layer" to facilitate
serialization of ResultSources.

In the process improve behavior/error messages of objects which
were deserialized without $schema re-attachment