xbuf: add xbuf_reserve
xbuf: rewrite append functions using reserve
xbuf: add xbuf_steal

This function is useful for building arrays.
skip GCC optimize pragma when using clang
ivs: depend on BigHash
OVSDriver: group table management
OVSDriver: basic forwarding for SELECT groups

This is sufficient for spreading traffic over a few ports. It does not yet
implement the watch_port logic to deal with link failures.
OVSDriver: initialize parsed key

This only became an issue once we started hashing the key for SELECT groups.
Merge into master from pull request #58:
basic LAG (SELECT group) support (https://github.com/floodlight/ivs/pull/58)