ovn-northd: Only run idl loop if something changed.

Before refactoring the main loop to reuse ovsdb_idl_loop_* functions, we
would use a sequence to see if anything changed in NB database to
compute and notify the SB database, and vice versa. This logic got
dropped with the refactor, causing a testsuite failure in the ovn-sbctl
test. Reintroduce the IDL sequence number checking.

Fixes: 331e7aefe1c6 ("ovn-northd: Refactor main loop to use ovsdb_idl_loop_*
functions")
Suggested-by: Numan Siddique <nusiddiq@redhat.com>
Signed-off-by: Joe Stringer <joe@ovn.org>
Signed-off-by: Justin Pettit <jpettit@ovn.org>
Tested-by: Thadeu Lima de Souza Cascardo <cascardo@redhat.com>
Acked-by: Ben Pfaff <blp@ovn.org>