Determine max key/value sizes

[benbjohnson]

Currently there are max key and value sizes but they were conservative guesses when the project started. They need to be updated to the actual limits.

[benbjohnson]

The current settings are safe and are above the range that's recommended to use. For example, large keys cause branch pages to grow since some keys are duplicated in branches. Really large values are probably a bad idea since there are a minimum two keys per page so changing one value will cause that value and the other value in the page to be copied to a new page (since Bolt is copy-on-write).