exchange: extract computation of pulled markers boundary in a function I expect massive experimentation on this specific aspect. so we better isolate it.

exchange: pull markers relevant to the pulled subset only With the command recently introduced we can select to pull only markers relevant to some nodes. We are now pull all markers for all node in the relevant subset. We'll try to pull less (just markers for node where local and remote marker diverge) later, but we need some marker discovery mechanism for that. which are not easy.

exchange: add a new method to pull markers It has both local and wire protocol implementation. This command does not requires escaping of the marker and are expecting to me highly faster than the pushkey method. The other main point is that command is to accept a `heads` and `common` argument to select to pull changeset only relevant to a set a of node. We are not using this capability yet but we'll do it soon