Renamed store/v2 -> store/v2alpha1

[aaronc]

store/v2 was not broken out into a standalone go module and we may want to do that at some point. Releasing it in the SDK as is will more or less prevent us for doing that because of the way the go module resolver works in terms of carving out new modules (see https://github.com/golang/go/wiki/Modules#is-it-possible-to-add-a-module-to-a-multi-module-repository). Thus would essentially force us to jump to store/v3.

Since store/v2 has not as far as we know gone into a stable release, we intend to rename it to store/v2alpha1 before 0.46 to all for a standalone store/v2 later, possibly with some API changes as well.