Improve discovery cooperation with EndpointStateActor #3642
Description
Endpoint::get_mapping_addr_and_maybe_start_discovery is a bit too simple right now. Or perhaps this is sufficient to get us to 1.0, let's decide which it is.
It has no knowledge of what state the EndpointStateActor has on the remote node. It uses EndpointStateActor::CanSend which is very primitive. So it does not really know if discovery should be started.
Maybe CanSend is sufficient for now, but needs to use more information like if there's an active connection or not, how old the addresses are that it has. Figure out how much smarter this needs to be.
The long-shot version of this is that the EndpointStateActor should be able to trigger discovery itself when it loses all paths to a remote. But that's probably more than we need right now.