Read some data from the handle at the specified offset.

template<
    typename MutableBufferSequence>
std::size_t read_some_at(
    uint64_t offset,
    const MutableBufferSequence & buffers);
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template<
    typename MutableBufferSequence>
std::size_t read_some_at(
    uint64_t offset,
    const MutableBufferSequence & buffers,
    asio::error_code & ec);
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