make laziness dynamic based on usage

Currently the tree has a lazy root by default, which reduces contention when modifications are random. But if modifications are always at the end of the tree, or clustered in a single subtree, we are back to serializing all writes.

To fix this, we should make inner nodes become lazy dynamically when they are being written too quickly. Something like this:

https://stack.convex.dev/rate-limiting

And then if the node isn't written to for a while, we can make it non-lazy again.

Potentially we would want a cron to make it non-lazy in case there are a ton of writes all at once and then a read-heavy workload afterwards.