ovsdb-idl: New function ovsdb_idl_create_unconnected().
This new function makes it possible to create an instance of the IDL
without connecting it to a remote OVSDB server. The caller can then
connect and disconnect using ovsdb_idl_set_remote(); the ability to
disconnect is a new feature.
With this patch, the ovsdb_idl 'session' member can be null whereas
previously it was always nonnull. The scattered changes throughout
ovsdb-idl are to cope with this new possibility.
An upcoming patch will introduce the first user of this new feature.