Replace the mutex-guarded callback model of peer.Conn with a per-peer
event loop that exclusively owns all mutable connection state. External
callers and transport workers post typed events into a non-blocking,
coalescing mailbox instead of contending on conn.mu:
- offers/answers coalesce to the newest message, a new offer flushes
queued candidates of the superseded session
- candidates are applied in arrival order from a bounded FIFO
- transport state changes are never dropped
- the blocking relay dial runs on a helper goroutine with a single dial
in flight; signaling I/O (offer/answer sends) runs off the loop
conn.mu now only guards the open/close lifecycle. Close posts a close
event and waits for the loop teardown; the loop also tears down on
engine context cancellation and releases resources of unprocessed
events.
Delete the Handshaker listener machinery (Listen loop, unbuffered
drop-on-busy channels, AsyncOfferListener with its double-processing of
the first offer), the never-wired dispatcher package and the unused
ICEMonitor.ReconnectCh. Fix a goroutine leak in the WG watcher test
that raced with tests mutating the package-level check timing vars.
5f98524e02