python: Monitor Database table to manage lifecycle of IDL client.

The Python IDL implementation supports ovsdb cluster connections.
This patch is a follow up to commit 31e434fc98, it adds the option of
connecting to the leader (the default) in the Raft-based cluster. It mimics
the exisiting C IDL support for clusters introduced in commit 1b1d2e6daa.

The _Server database schema is first requested, then a monitor of the
Database table in the _Server Database. Method __check_server_db verifies
the eligibility of the server. If the attempt to obtain a monitor of the
_Server database fails and a cluster id was not provided this implementation
proceeds to request the data monitor. If a cluster id was provided via the
set_cluster_id method then the connection is aborted and a connection to a
different node is instead attempted, until a valid cluster node is found.
Thus, when supplied, cluster id is interpreted as the intention to only
allow connections to a clustered database. If not supplied, connections to
standalone nodes, or nodes that do not have the _Server database are
allowed. change_seqno is not incremented in the case of Database table
updates.

Acked-by: Numan Siddique <nusiddiq@redhat.com>
Signed-off-by: Ted Elhourani <ted.elhourani@nutanix.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

python/ovs/reconnect.py

@@ -344,6 +344,9 @@ class Reconnect(object):
                 else:
                     self.info_level("%s: error listening for connections"
                                     % self.name)
+            elif self.state == Reconnect.Reconnect:
+                self.info_level("%s: connection closed by client"
+                                % self.name)
             elif self.backoff < self.max_backoff:
                 if self.passive:
                     type_ = "listen"