ovsdb-idl: New function ovsdb_idl_create_unconnected().

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.

Signed-off-by: Ben Pfaff <blp@ovn.org>