ovsdb-idl: Fast resync from server when connection reset.

ovsdb-idl: Fast resync from server when connection reset.

Use monitor_cond_since to request changes after last version of local
data when connection to server is reset, without clearing the local
data. It falls back to clearing and repopulating all the data when
the requested id cannot be fulfilled by the server.

Test result at ovn-scale-test environment using clustered mode:
- 1K HVs (ovsdb clients)
- 10K lports

Without the patch it took 30+ min for the SB ovsdb-server to calm down
and HVs to stablize the connectin and finish syncing data.

With the patch there were no noticible CPU spike of SB ovsdb-server,
and all HVs were in sync with SB within 1 min, which is the probe
interval set in this test (so it took at most 1 min for HVs to notice
the TCP connection reset and reconnect and resync finished immediately
after that).

Reported-at: https://mail.openvswitch.org/pipermail/ovs-discuss/2018-September/047457.html
Signed-off-by: Han Zhou <hzhou8@ebay.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>