mount: batched announcer + pooled peer conns for mount-to-mount RPCs
* peer_announcer.go: non-blocking EnqueueAnnounce + ticker flush that
groups fids by HRW owner, fans out one ChunkAnnounce per owner in
parallel. announcedAt is pruned at 2× TTL so it stays bounded.
* peer_dialer.go: PeerConnPool caches one grpc.ClientConn per peer
address; the announcer and (next PR) the fetcher share it so
steady-state owner RPCs skip the handshake cost entirely. Bounded
at 4096 cached entries; shutdown conns are transparently replaced.
* WFS starts both alongside the gRPC server; stops them on unmount.
Chris LuandGitHub
73f10fa528