Allow configuring max chunk size

[lulf]

Changes the magic since it breaks existing on-disk format

[Dirbaio]

• i'd make MAX_CHUNK_SIZE the min of the old formula and the user-configured value. This way there's no RAM "wasted" if page_size - headers is smaller.
• to make sure ekv doesn't attempt to mount an old disk image at all, change the magics in MetaHeader, DataHeader. Changing just the chunk magic will make it mount it but see all pages as empty.

[lulf]

• i'd make MAX_CHUNK_SIZE the min of the old formula and the user-configured value. This way there's no RAM "wasted" if page_size - headers is smaller.

I think this would require using const_trait_impl in order to get const min().

[lulf]

Nevermind, just did it manually :)

[Dirbaio]

🚀 thank you!