Closes #6087: Cache prefix depth & children count

[jeremystretch]

Fixes: #6087

• Ditch annotate_tree() PrefixQueryset method; replace with optional annotate_hierarchy()
• Add depth and children fields on ipam.Prefix to cache depth and child counts
• Add a second migration to efficiently update these values for existing deployments
• Introduce signal receivers to automatically update these values on post_save and post_delete
• Add get_parents() and get_children() convenience methods on ipam.Prefix

TODO

• Management command to manually rebuild the prefix hierarchy
• Extend depth and children query filters