RtcLogger: Use MS_DUMP() instead of std::cout()

[jmillan]

Closes #1551

Output looks like:

RTC::RtcLogger::Log() | {"timestamp": 4518452978, "recvTransportId": "fe2bf4c2-f939-4d51-b8fd-73ba30b3a196", "producerId": "3d47591c-1ffc-40ce-87c9-b2636ee39df9", "recvRtpTimestamp": 1962299018, "sendRtpTimestamp": 0, "recvSeqNumber": 12595, "sendSeqNumber": 0, "discarded": true, "discardReason": 'RecvRtpStreamNotFound'}