ovsdb: Change the way connection duration time is reported in Manager table.

ovsdb: Change the way connection duration time is reported in Manager table.

Commit 0b3e7a8b71 (ovsdb-server: Write manager status information to Manager
table.) attempted to provide managers with the ability to debug manager-related
connection problems, but it turns out that reporting "time_in_state" is not
very useful, because the state is constantly changing. What people really want
is the time each manager has been connected or disconnected, depending on the
current connection state.

Replace "time_in_state" key with "time_connected" and "time_disconnected"
keys. Only one exists at a time, and time is in seconds.

Bug #4833.