this lets us get rid of a lot of extra switch statements - in a lot of
places we dispatch on the btree node type, and then the key type, so
this is a nice cleanup across a lot of code.
Also improve the on disk format versioning stuff.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
It's now possible to create and use a filesystem on a 512k device with
4k buckets (though at that size we still waste almost half to internal
reserves)
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
