Removing a TransfVec also removes it's chunks

(and never the underlying chunks)
Cleanup Vec toEnum & Rollups

Deprecated code removed.  Constraint is tested & throws IAE if violated.
Rollups use the much simpler Futures API, the CPS style call was not
actually used by anybody.  Add a missing "already got rollups" test.
Handle OutOfMemory for Direct ByteBuffers

Just stall & retry; probably the buffers get returned shortly.
Converts an OOM-death situation into a slowdown or hang.