When a message is received, the initial MODACK value used should be
the same value currently used by leaser. This makes sure the leaser
will wake up and extend the ACK deadline before the initial one
expires.

If flow_control.max_duration_per_lease_extension is set to too low a
value, it is adjusted to the minimum ACK deadline.

The ACK deadline is dynamically being adjusted based on ACK data,
but an updated deadline value must not be used for the new messages
that arrive while the leaser thread is still sleeping.

Doing so could result in the messages' ACK deadlines expiring before
the leaser wakes up and has a chance of extending the deadlines for
these messages. An updated ACK deadline value for new messages is thus
only used when the leaser starts using it, too.