Add SO_TIMESTAMP support for datagram channels

[klaaspieter]

Motivation:

NIO supports ECN, packet info, and segmentation metadata but not timestamps. Userspace timestamps are imprecise due to scheduler latency between packet arrival and recvmsg. SO_TIMESTAMP captures the exact kernel receive time with microsecond precision via control messages, which is critical for accurate RTT measurements and latency monitoring.

Modifications:

• Add timestamp field to AddressedEnvelope.Metadata
• Add TimestampOption channel option
• Parse SCM_TIMESTAMP control messages in ControlMessageParser
• Add init(ecnState:packetInfo:segmentSize:timestamp:) to Metadata
• Add test coverage

Result:

Datagram channels can enable .receiveTimestamp to receive kernel timestamps in AddressedEnvelope.Metadata.timestamp as a Double (seconds since epoch with microsecond precision).

[Lukasa]

Thanks for this patch! This mostly looks really good, I've left a few notes in the diff.

[klaaspieter]

@Lukasa the API breaking change made sense because I changed a constructor. What is the best way to handle this? I can add a new constructor with the new argument, keeping the old one unchanged. I can also make the default value for timestamp be nil.

[glbrntt]

@Lukasa the API breaking change made sense because I changed a constructor. What is the best way to handle this? I can add a new constructor with the new argument, keeping the old one unchanged. I can also make the default value for timestamp be nil.

You'll need to add a new init and keep the old one unchanged. Adding a default value to an existing function is actually an API break.