Ignore delay_resp messages from foreign masters.

When a new master appears, it will start to respond to our delay_req
messages. Make sure we process only responses from our current master
before switching to the new master.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Don't calculate delay with old master's sync time stamp.

When a new master is selected, drop the old sync time stamp to prevent
calculating invalid delay in case delay_resp will be received before
first sync from the new master.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>