bgpd: Convert the bgp_advertise_attr->adv to a fifo (backport #14554)

[mergify]

BGP is storing outgoing updates in a couple of different fifo's. This is to ensure proper packet packing of all bgp_dests that happen to use the same attribute.

How it's all put together currently: On initial update BGP walks through all the bgp_dest's in a table. For each path being sent a bgp_advertise is created. This bgp_advertise is placed in fifo order on the bgp_synchronize->update queue. The bgp_advertise has a pointer to the bgp_advertise_attr which is associated iwth the actual attribute that is being sent to it's peer. In turn this bgp_advertise is placed in a fifo off of the bgp_advertise_attr structure. As such as we have paths that share an attribute, the path/dest is placed on the bgp_syncrhonize->update fifo as well as being placed on the fifo associated with the advertised attribute.

On actual creation of a packet. The first item in the bgp_synchronize->update fifo is popped. The bgp_advertise_attr pointer is grabbed, we fill out the nlri part of the bgp packet and then walk the bgp_advertise_attr fifo to place paths/dests in the packet. As each path/dest is placed in the packet it is removed from both the bgp_synchronize->update fifo and the bgp_advertise_attr fifo.

The whole point of this change is to switch the *next, *prev pointers in the bgp_advertise structure with a typesafe data structure.

---

This is an automatic backport of pull request #14554 done by Mergify.