Follow up to #56961:
We can be a little more efficient than just serializing at the IO loop by serializing
only when we flush to a channel. This has the advantage that we don't serialize a long
queue of messages for a channel that isn't writable for a longer period of time (unstable network,
actually writing large volumes of data, etc.).
Also, this further reduces the time for which we hold on to the write buffer for a message,
making allocations because of an empty page cache recycler pool less likely.
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