Because we want to be able to read messages from async sources (like a
tcp socket), we need to have at least async header parsing logic, so
that we can correctly determine how many bytes to await to parse each
message, so it makes sense for the entire message parsing functions
to be async.
Because we perform message serialization into async readers and writers
in the context of sending messages over the network, code using these
functions is more clear with these names.
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