Added assertion in hllSparseAdd() when promotion to dense occurs.

If we converted to dense, a register must be updated in the dense
representation.
Correctly replicate PFDEBUG GETREG.

Even if it is a debugging command, make sure that when it forces a
change in encoding, the command is propagated.
Mark PFDEBUG as write command in the commands table.

It is safer since it is able to have side effects.
PFMERGE fixed to work with sparse encoding.