refactor(multipath): Store Watcher, not Watchable on Connection

[Frando]

Description

Currently the Connection stores a Watchable for the open and selected paths. This means, though, that a watcher stream for the open paths will only close once the last connection is dropped, not once the connection is closed. That is bad API and will lead to confusion.

In an initial version, I swapped the Watchables for a n0_watcher::Map<(Direct, Direct)> but this increases the size of the Connection struct from ~80 to ~700 bytes, because it would store both the two unmapped values and the mapped final value inline. It is also completely unneeded, because we'd clone out the watcher anyways on Connection::watch and get or stream on the watcher would refetch the value.

Instead, this now uses n0-computer/n0-watcher#31 to only store a weak pointer to the watchable's shared state without keeping it alive, and then creates the PathInfos on demand from that. The stats are now fetched while iterating, so if you're e.g. only checking the number of paths, the stats won't be fetched at all.

We fetch the paths through upgrading the handle in the next method, so that we can return None if the connection was dropped in the meantime (the alternative would be a fallible PathInfo::stats method)

Breaking Changes

Notes & open questions

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All breaking changes documented.
  • List all breaking changes in the above "Breaking Changes" section.
  • Open an issue or PR on any number0 repos that are affected by this breaking change. Give guidance on how the updates should be handled or do the actual updates themselves. The major ones are:
    • quic-rpc
    • iroh-gossip
    • iroh-blobs
    • dumbpipe
    • sendme

[Frando]

Closing this - n0-computer/n0-watcher#31 is likely not going to be merged and #3632 was merged for now.