BGP should set DSCP bits

RFC 4271 Appendix E:
```
   If a local system TCP user interface supports setting the DSCP field
   [RFC2474] for TCP connections, then the TCP connection used by BGP
   SHOULD be opened with bits 0-2 of the DSCP field set to 110 (binary).
```

This seems simple enough to do via setsockopt, so we should consider adding support for this (default value w/ ability to override).