phase: use a binary phase part to push through bundle2 (BC)
The part we are using for pull is now used for push too. As we no longer use
pushkey, pushkey hooks are no longer triggered. This is an obvious backward
incompatible change. We could artificially trigger the pushkey hook within the
bundle2 part, but this seemed too hacky to me.
An option would be to disable by default this new mechanism for a couple of
versions to help people migrate to txnclose-phase. I took the liberal and
optimistic path to just turn it on by default directly.
Push no longer triggers a pushkey hook when updating phases. Use the new `txnclose-phase` and `txnclose-phase` hooks instead. (Applies when both server and client use version 4.4 or above).