This patch adds the view_updates::generate_update() function to
generate view updates given a base row update and the corresponding,
pre-existing row. This function will decide which of the previously
introduced functions to call based on whether there is a pre-existing
row and whether there exists a regular base column that's part of the
view's PK.
Signed-off-by: Duarte Nunes <duarte@scylladb.com>