When large binary protocol packets were handled, a part of the data was
replaced with a non-existing PS ID.
The replacement of the client PS ID to the internal ID and the replacement
of the internal ID to the server specific ID must only be done if a large
packet is not being processed. This can be done on the router level
without adding knowledge of large packets to the RWBackend class.
A specific function, RWBackend::continue_write, was added to make it clear
that the buffer being written is a part of a larger query. The base class
Backend::write could be used but its usage is not self-explanatory.
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