v1.7.0-alpha.6

• #100 Migrated to Tokio Runtime.
• [BUG-FIXED] #105 Fixed "Too many open files" in UDP relay.
• [BUG-FIXED] Fixed bug while starting UDP relay. While starting server with plugins, it should not change the listening addresses for UDP relay, which are only for TCP relay.
• [BUG-FIXED] #106 Server should not panic if accepted socket closed right after accept().
• Implemented a new ssdns server, which can serve as a DNS server and proxy DNS queries via ShadowSocks' UDP relay.
• Replaced ToSocketAddrs with trust-dns
• Uses impl Trait for functions
• Updated dependencies