ovsdb-idl: Tolerate initialization races for singleton tables.

ovsdb-idl: Tolerate initialization races for singleton tables.

By verifying that singleton tables (that is, tables that should have exactly
one row) are empty when they emit transactions that insert into them,
ovs-vsctl and similar tools tolerate initialization races, where more than one
client at a time tries to initialize a singleton table.

The upshot is that if you create a database and then run multiple ovs-vsctl
(etc.) commands against it in parallel (without first initializing it
serially), then without this patch sometimes you will sometimes get failures
but this patch avoids them.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Alin Gabriel Serdean <aserdean@ovn.org>