Reduce latency of write operations

As described in [this](https://github.com/boostorg/redis/issues/246#issuecomment-2927909878) comment there is an easy way to reduce latency of write operations

> We can avoid copying payloads [here](https://github.com/boostorg/redis/blob/0c8c6fcc095ff103e9adda708c1efb06fabfe08d/include/boost/redis/impl/multiplexer.ipp#L158) when the connection has only one ongoing `async_exec`. The multiplexer `get_write_buffer()` function could then return a `string_view` pointing directly to the request internal buffer instead of a copy of it, which would then be used [here](https://github.com/boostorg/redis/blob/0c8c6fcc095ff103e9adda708c1efb06fabfe08d/include/boost/redis/connection.hpp#L199).