bookmark: introduce in advance a variant of the exchange test We are about to introduce a new way to push bookmark to server. We introduce the test variant before actually updating the exchange to help the output changes to stand out when it happens.

push: move bundle2-pushkey based bookmarks exchange in its own function We are about to introduce an alternative way to push bookmark over bundle2.

bookmark: add pushkey hook compatiblity to the bundle2 part Currently, pushing a bookmark update triggers a pushkey hooks. It is likely that users in the wild use such hooks to control bookmark movement. Using a non push-key mechanism to exchange bookmark means these hooks are no longer called, possibly breaking existing users setup. So we add explicit call to the pushkey hooks in the handling of the bundle2 part. This behavior can be disabled with a new config knob: 'server.bookmarks-pushkey-compat'.