FindAllLocalBranches: support paginated responses

Pagination has always been ad hoc within Gitaly, and there was no convergence to a standard. This change creates a structure around this, which an implementation for one RPC to try it.

This change introduces a structure for pagination in the shared.proto, and allows FindAllLocalBranches to respond correctly to the new fields. Which required changes to the lines.Send() behaviour.

Most of the behaviour is captured in the SenderOpts in the lines package, and depends on a PageToken to skip results. Page tokens allow for a generic interface in other RPCs too, and for this RPC its the last branch name.