ovn-northd: Revert "ovn-northd: Only run idl loop if something changed"

ovn-northd: Revert "ovn-northd: Only run idl loop if something changed"

This reverts f20396e051ea4fd623bfc022cc78d9bd52a850e5

f20396e.. was required to fix the failing ovn-sbctl testsuites.
commit fb496f92ca1eeead8760b5cdfd857165f64deadf addresses these failing
test cases in a proper way.

With commit f20396e.. its possible that commit to the southbound db is
delayed. When ovnnb_db_run is called, and if ctx->ovnsb_txn is NULL,
ovnnb_db_run returns immediately without generating the logical flows.
ovnnb_db_run is not called again until the northbound db seqno changes.

Signed-off-by: Numan Siddique <nusiddiq@redhat.com>
Signed-off-by: Russell Bryant <russell@ovn.org>