Breaking changes in switching from uint64 to atomic.Uint64

[just-hms]

Using go version <1.19 leads to compiler error:

/github.com/tidwall/[email protected]/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 where there is no go 1.19 requirements either.

[tidwall]

thanks. I bumped it to 1.19.