Prefer Using Source Time if Provided

[maxsharabayko]

The source time for a message can be provided using the SRT_MSGCTRL structure:

typedef struct SRT_MsgCtrl_
{
   // …
   int64_t srctime;             // Source time, in microseconds since SRT internal clock epoch
   // …
} SRT_MSGCTRL;

Currently, inside SRT a message gets two timestamps:

1. Submission timestamp (tsOriginTime), which is used for:
   • SND Too-Late Packet Drop;
   • TTL Message Drop (not to be confused with IP_TTL);
   • SND buffer timespan (stats).
2. Source timestamp if provided (tsSourceTime)
   • Data Packet Timestamp

This PR keeps only one timestamp for everything (source time if provided, otherwise submission time).

Note: Extracted from #2180.