[core] Drop undecrypted packet based on sequence number.

[maxsharabayko]

The PR addresses issue #2626.

When packet decryption fails, the fields of the packet cannot be trusted, and therefore used for dropping the whole message.
Given that SRT only allows AEAD in live streaming configuration at the moment, it is ok to drop a packet from the RCV buffer based on the packet sequence number used to insert it into the buffer. In the live streaming configuration, one packet represents a single message, thus by dropping one packet the whole message is dropped.

This approach should be good enough for v1.5.2 and for live streaming configuration in general.
A more universal approach would require removing a packet from the RCV buffer (without dropping( and adding the packet sequence number to the RCV loss list (see PR #2649). Also, the RCV buffer state would have to be reset back to the one before the packet has been inserted.
An alternative approach could be to check if the RCV buffer will accept the packet, decrypt and insert it into the buffer only if decryption succeeds.
In any case, such changes would require a more noticeable restructuring of the source code. Something for v1.6.0.