Key changes:
- No mutex for every udp package: replace syncs.Map with regular map for udpTargetConns
- Use socksAddr as map key for better type safety
- Add test for multi udp target
Updates #7581
Change-Id: Ic3d384a9eab62dcbf267d7d6d268bf242cc8ed3c
Signed-off-by: VimT <me@vimt.me>