Reloads checkout after registration which fixes #3612, removes some notices preventing login cookie being set, moves cart check for #3162

No perfect solution for #3162. DB locking would probably cause more
issues that its worth, we cannot reliably use transactions as some
clients still use MyIsam. Best to accept orders and deal with stock
conflicts after the fact.
Merge branch 'master' of https://github.com/woothemes/woocommerce