[#1959] Changes in heartbeat and lease updates

[#1959] Changes in heartbeat and lease updates

Previously, we'd assume that the communication with the partner is ok
when lease update was sent successfuly. In that case, we'd not send a
heartbeat. Under a heavy load, we could delay heartbeats for a long time.
Sending heartbeats is actually important because the heartbeat response
conveys information about the partner state. We want to have up-to-date
information about partner's state.