ovsdb-server: Write manager status information to Manager table.

ovsdb-server: Write manager status information to Manager table.

This commit makes the status of manager connections visible via the Manager
table in the database.  Two new columns have been created for this purpose:
'is_connected' and 'status'.  The former is a boolean flag, and the latter is a
string-string map which may contain the keys "last_error", "state", and
"time_in_state".

Requested-by: Keith Amidon <keith@nicira.com>
Reviewed by: Ben Pfaff.
Feature #3692.