docs/tutorial.md: add section about memory management best practices

[denesb]

Memory management in a seastar application is a very important topic, as seastar applications are usually long running server applications that want to maximize the usage of memory as well as be resilient to memory fragmentation. Add a section on this to the docs/tutorial.md based on the lessons we learned at ScyllaDB. In a nutshell: when large buffers are needed prefer storing data in a chunked form, each chunk being of some 2^k size in the range of kilobytes. At ScyllaDB we chose our magic buffer size to be 128k. This is by no means the perfect or the only valid such buffer size, we just found this size to work quite well for us.