Do not set a Close finalizer on accepted streams (#116)

[youdie006]

Fixes #116.

Problem

AcceptStream registers a runtime finalizer that calls Close on the returned *Stream wrapper:

wrapper := &Stream{stream: stream}
runtime.SetFinalizer(wrapper, func(s *Stream) {
    s.Close()
})

The canonical server usage is stream, _ := session.AcceptStream(); go io.Copy(dst, stream). io.Copy dispatches to WriteTo, which is promoted from the embedded *stream, so during the copy the outer *Stream wrapper is reachable only through io.Copy's now-dead argument. GC then collects the wrapper, runs the finalizer, and closes the stream mid-transfer; the frames that arrive afterwards are silently dropped on recvLoop's closed-stream path, so the receiver just comes up short with no error.

OpenStream already has the identical finalizer disabled with the comment // NOTE(x): disabled finalizer for issue #997, so AcceptStream was simply left inconsistent.

Fix

Remove the finalizer from AcceptStream (matching OpenStream), and drop the now-unused runtime import.

Test

Added TestAcceptStreamNoCloseFinalizer: a server accepts a stream and drains it with a bare go io.Copy(counter, stream) (so the wrapper is not retained) while the test forces GC, and a client sends 1 MiB. It asserts the server drains all 1 MiB. Red-green verified with go test: before the fix the server drains only ~224 KiB before the finalizer closes the stream (the test fails at its deadline); after, it drains the full payload in ~30 ms. go build, go vet and the full go test ./... suite pass.