topology/switches: duplicate datapath connections handling

The way to handle multiple connections from the same datapath I took
is mostly the same as in dpset, but it's always good to reinstall
the LDAP Packet-In flows even while switch tries to reconnect to us.

Signed-off-by: Wei-Li Tang <alextwl@xinguard.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>