Allow SQL expressions to be set on PK columns

Allow SQL expressions to be set on PK columns

Removes an unnecessary transfer of modified PK column
value to the params dictionary, so that if the modified PK column
is already present in value_params, this remains in effect.  Also
propagate a new flag through to _emit_update_statements() that will
trip "return_defaults()" across the board if a PK col w/ SQL expression
change is present, and pull this PK value in _postfetch as well assuming
we're an UPDATE.

Change-Id: I9ae87f964df9ba8faea8e25e96b8327f968e5d1b
Fixes: #3801