Speed up cluster gossip processing #3441

* Check VectorClock for common case first
* Cache hashCode for VectorClock.node and UniqueAddress

I've replayed the gossip events for a couple of the nodes from the large cluster stress run
(only events where the packed version was larger than 8k) and same gossip was by far the
most common case (81%).

The culprit is the merging of the seen tables. It should probably be rewritten but this
speeds up the processing in the meantime (see t the meantime (see ticket 3442)

On my machine the average time for same gossip dropped from 20ms to 7ms, and merge gossip
from 14ms to 9ms. Both older and newer take below 1ms.