This diff adds a structure for remembering the nodes waiting for a
proof, and bind them as soon as this proof is knwon (i.e. a peer with
this proof is created).
The nodes are added upon reception of a avahello message. If the
associated proof is already know, they are added to the peer
immediately, otherwise they are added to the pending list and bound to
the peer when the proof is later downloaded and verified.
This feature is hidden behind a dedicated flag so one can choose to only
bind manually submitted nodes.
Depends on D9700.
Ref T1634.