ovsdb: Enforce immutability of immutable columns.

ovsdb: Enforce immutability of immutable columns.

OVSDB has always had the ability to mark a column as "immutable", so that
its value cannot be changed in a given row after that row is initially
inserted.  However, we discovered recently that ovsdb-server has never
enforced this constraint.  This commit implements enforcement.

Reported-by: Paul Ingram <paul@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Kyle Mestery <kmestery@cisco.com>