Rel 50, 52, 57 revisited

Rel 50: (solution c) Semantics of ExpiryTime:
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Proposal:

ExpiryTime indicates the ultimate date after which the receiver RMP MUST
not deliver a received message to the application (delivery being
abstractly defined as the transfer of responsibility for processing a message, 
from RMP to the next layer.)

After a message has been sent for the first time, ExpiryTime in a message 
MUST NOT be modified in any case by the Sender, when resending the message:
two messages with same ID (duplicates) MUST also have same ExpiryTime.

When a message expires on the Sender side before being sucessfully sent,
a Sender RMP MUST NOT send it or resend it, and must communicate a delivery failure 
to the Sender application.

NOTES: 

- Given the above definition of ExpiryTime, in case duplicate elimination is required, 
when a received message is processed, it is sufficient to only check for its duplicates 
among IDs of past messages that have not expired yet at the time of the duplicate check.