Implement a special update generator for add-path send all.

Implement a special update generator for add-path send all.

The generic add-path code up_generate_addpath() reevaluates everything
since this is the simplest way to select the announced paths. For add-path
all this is overkill since there is no dependency between prefixes and so
individual prefixes can be handled more efficently.

Extend rde_generate_updates() to pass the current newbest and oldbest
prefixes (for the selected best path) but now also include newpath and
oldpath (which is the prefix that is added/removed/modified).
If newpath or oldpath is set then a single prefix was altered and
up_generate_addpath_all() can just remove or add this prefix.
If newpath and oldpath are NULL than the full list based on newbest
needs to be inserted and any old path/prefix removed in the process.

This improves update generation performance on big route collectors using
add-path all substantially.

OK tb@