[core] Use SND buffer delay for TL Packet Drop instead of the timespan

[maxsharabayko]

1. Use SND buffer delay for TL Packet Drop

Use SND buffer delay for TL Packet Drop instead of the timespan between the first and the last packet in the buffer.

Sender dropping logic was applied if the sender buffer timespan exceeds drop_threshold_ms.
Instead, the delay of the oldest packet in the buffer should be used (Tnow - OldestPacketTS).
This also correlates with the usage of the source time if provided to calculate the timestamp, which was not the case previously (PR #2185).

As before, only packets older than Tnow - drop_threshold_ms are dropped by the sender.

Fixes #898.

2. Added missing mutex lock to CSndBuffer::readData(..) function.

For some reason the lock was missing from the CSndBuffer::readData(CPacket& w_packet, ..).