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

exchange: add the pushmarker wireproto command to simple4server This will allow simple server side support. (yes, code duplication is bad, I won't do it too much mom)