A MessagePort can be transferred while it already has local queued
state such as incoming messages drained from its transport,
outgoing messages posted before a transport exists, and a pending
shutdown to apply once the port is enabled.
Serialize and restore that state as part of transfer so it moves with
the port instead of being left behind on the old transport.
Also mark transports that are being transferred so shutdown of the old
endpoint during handoff is not reported as peer EOF. That shutdown is
part of moving the transport to the new owner, not peer disconnected.
Co-Authored-By: Alexander Kalenik <kalenik.aliaksandr@gmail.com>
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