Mark file alignments as const

[xemul]

There are four of them currently, they are set when file is constructed and never change then, but are not marked as const. The main (and the only) obstacle is that the values are initialized in inheritants' constructors bodies. So the main change in this set is moving the initialization into file_impl's initializer lists

[avikivity]

Didn't we leak these members? Will layered file implementations break?

I see it in scylladb-enterprise mirrored_file_impl.

[xemul]

Didn't we leak these members?

I don't think we did -- they are plain integers

Will layered file implementations break?

It will:

in_memory_file_impl::in_memory_file_impl(std::shared_ptr<foreign_ptr<std::unique_ptr<in_memory_data_store>>> data)
        : _data(std::move(data))
{
    // We're not doing any actual dma transfers and we do not require
    // any specific alignment.
    // The following alignment parameters are solely an optimization.
    _memory_dma_alignment = seastar::cache_line_size;
    _disk_read_dma_alignment = in_memory_data_store::chunk_size;
    _disk_write_dma_alignment = in_memory_data_store::chunk_size;
}

someone will need to patch it to use : file_impl(...) delegation