ovsdb-server: Report time since last connect and disconnect for each manager.

ovsdb-server: Report time since last connect and disconnect for each manager.

Only the time connected (if connected) or disconnected (if disconnected) is
currently reported for each manager. Change to reporting both in seconds since
the last connect and disconnect events respectively. An empty value indicates
no previous connection or disconnection.

This can help diagnose certain connectivity problems, e.g. flapping.

Requested-by: Peter Balland <peter@nicira.com>
Bug #4833.