Use diff_addr_store.py in the master flow plugin, to share the same code
with FWUp to handle updating the address sets.

Don't leave old values in the sets when the plugin starts and does a
full update. They might be gone by now, so do a full replace of the set.

Use socketpair to talk with the ipset command instead of pipe. For one,
we couldn't use send with pipe. For another, we are able to log its
output and see if something went wrong.

Use the new variadic functions for parsing & rendering of data, making
the code shorter in the process.

They allow to parse/render multiple elements at once, doing all the
necessary size checks.

Use it so the error messages are sent to the server.

Provide hooks and commands to create and update the sets.

• The hooks don't need the userdata parameter, that one is passed inside
  the storage parameter.
• Have a hook after complete replacement as well as before, to allow
  working on a copy and atomically swapping instead of just working or
  real data all the time.

• New module containing the set types and definitions.
• Each set gets its own function to convert its members (addresses) to
  strings that can be fed to the ipset utility.

Provide hooks for adding, removing and clearing of addresses.

If the set is present in both old and new config, migrate its content
and state, so we don't have to re-download everything.

Add a function to copy content of one address store into another.

Mark the import variable as possibly unused, suppressing a warning in
case the variable is created but not imported into and unused.

Be able to reload pluglibs after the plugin crashes and is reloaded.