[core] Refactoring: added packUniqueData(..) function

[maxsharabayko]

The code related to forming an original data packet for sending has been moved to a separate function packUniqueData(..).
The encryption of the payload has been moved inside that function as well, as it is only needed to encrypt the original payload. Once encrypted, it is stored so in the sender buffer, thus the second encryption call is needed when the packet is being retransmitted.
Moving the encryption inside the packUniqueData(..) also means that packet timestamp and destination socket ID are not yet set in the header of the packet. However, only the packet sequence number is used as the counter for the AES encryption, therefore this change is safe.

Changes extracted from PR #2180.

Tests

Checked encryption and decryption including KM refresh using srt-xtransmit:

(receiver)
./srt-xtransmit receive "srt://:4200?passphrase=abcdefghij" --enable-metrics -v

(sender)
./srt-xtransmit generate "srt://127.0.0.1:4200?passphrase=abcdefghij&kmrefreshrate=1000&kmpreannounce=200" \
                         --enable-metrics --sendrate 10Mbps --duration 60s -v