Optimize the gossip processing

This pull contains two commits
* =clu Optimize the vector clock comparison
* !clu Convert the seen table into something more efficient

The first one cuts the normal vector clock comparison time roughly in half
(it's backwards compatible).

The second exploits the fact that we only care about who has seen this exact
gossip version and does away with the mapping between nodes and the gossip version
that they have seen for a set of nodes which have seen this exact gossip version.
The gossip processing for the normal case (same gossip) drops from ~7ms to sub ms for
a 100 node cluster. The only thing that takes any time now is the merging of vector clocks
when you have a gossip conflict(which is rare), but that's still in the 2ms range for a
100 node cluster.