[BUG] Handle clock drift and timestamp carryover on idle connections

[maxsharabayko]

If there is a connection, but no data is being sent, peers will be exchanging with keepalive messages.

One thing that can happen is a timestamp carryover (after 1h10m).
Timestamp carryover on an idle receiver was addressed in PR #1895, but clock drift was potentially still there, so the PR was not accepted.
The same is true for a sender: it does not keep track of the receiver's timestamps. It may become a problem if a sender eventually starts receiving data packets (bidirectional transmission).

The second thing is that the drift tracer does not receive new samples, and does not track clock drift.
True for the sender and for an idle receiver.

SRT v1.4.3, v1.4.2 and prior.

To partially address the issue, the KEEPALIVE packet may be extended with synchronization clock timestamp.