ovsdb-idl: Return correct seqno from ovsdb_idl_db_set_condition().

ovsdb-idl: Return correct seqno from ovsdb_idl_db_set_condition().

If an IDL client sets the same monitor condition twice, the expected
seqno when the IDL contents are updated should be the same for both
calls.

In the following scenario:
1. Client calls ovsdb_idl_db_set_condition(db, table, cond1)
2. ovsdb_idl sends monitor_cond_change(cond1) but the server doesn't yet
   reply.
3. Client calls ovsdb_idl_db_set_condition(db, table, cond1)

At step 3 the returned expected seqno should be the same as at step 1.
Similarly, if step 2 is skipped, i.e., the client calls sets
the condition twice in the same iteration, then both
ovsdb_idl_db_set_condition() calls should return the same value.

Fixes: 46437c5232bd ("ovsdb-idl: Enhance conditional monitoring API")
Signed-off-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>