refactor(iroh): Factor out socket-related state & construction into magicsock::SocketState and ActorSocketState (#3203)

## Description

This factors out the socket & related state from `MagicSock` into a
`SocketState` struct, as well as from the `Actor` struct into an
`ActorSocketState` struct.

This helps reduce line noise in the `magicsock::Handle::with_name`
function related to all the `#[cfg(not(wasm_browser))]`.

In order to do this I needed to slightly modify how `UdpConn` is used.
We now work *more* with `Arc<UdpSocket>` rather than `UdpConn`, and
treat `UdpConn` only as a wrapper to allow us to implement
`quinn::AsyncUdpSocket` for `Arc<UdpSocket>`.
I've also moved the socket binding logic from `udp_conn.rs` to
`magicsock.rs` and cleaned it up.

The main difference between `SocketState` and `ActorSocketState` is that
the former stores `UdpConn` and the latter stores `Arc<UdpSocket>`, as
the former uses its sockets only to implement methods from
`AsyncUdpSocket` for `MagicSock`, whereas the latter uses it for a bunch
of things, including sending disco messages and port mapping.

Finally, I also went ahead and renamed any `pconn4` and `pconn6` names
to say `socket`, `socket.v4` or `socket.v6`, as I find that would make
more sense.

## Change checklist
<!-- Remove any that are not relevant. -->
- [x] Self-review.
- [x] Documentation updates following the [style
guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text),
if relevant.

Author: matheus23

iroh-net-report/src/lib.rs

@@ -37,7 +37,6 @@ use tracing::{debug, error, info_span, trace, warn, Instrument};
 mod defaults;
 #[cfg(not(wasm_browser))]
 mod dns;
-#[cfg(not(wasm_browser))]
 mod ip_mapped_addrs;
 mod metrics;
 #[cfg(not(wasm_browser))]
@@ -65,7 +64,6 @@ pub mod portmapper {
     }
 }
 
-#[cfg(not(wasm_browser))]
 pub use ip_mapped_addrs::{IpMappedAddr, IpMappedAddrError, IpMappedAddresses, MAPPED_ADDR_PORT};
 pub use metrics::Metrics;
 pub use options::Options;