Breaking changes in switching from uint64 to atomic.Uint64

Using go version <1.19  leads to compiler error:

```
/github.com/tidwall/btree@v1.5.1/btreeg.go:41:17: undefined: atomic.Uint64
```

I suggest to switch to 1.19 in go.mod and to explicitly say to the users that this package is not backward compatible.

Found this issue using [buntDB](https://github.com/tidwall/buntdb) where there is no go 1.19 requirements either.