serf: Lamport clock Witness retries if CAS fails
serf: Lamport clock Witness retries if CAS fails

/cc @armon - I couldn't think of a way to sanely test this, though
there must be some probablistic test we can do. At any rate, CAS
failures are potentially possible, so this should be good, but would
appreciate your eyes on it.

samaitra/serf,FollowMyDev/serf,yonglehou/serf,mapuri/serf,samaitra/serf,samaitra/serf,hashicorp/serf,dalehamel/serf,patrickviet/serf,mapuri/serf,hashicorp/serf,patrickviet/serf,patrickviet/serf,pmenglund/serf,pmenglund/serf,FollowMyDev/serf,mbrukman/serf,samaitra/serf,yonglehou/serf,pmenglund/serf,jmptrader/serf,mapuri/serf,hashicorp/serf,minhkiller/serf,patrickviet/serf,jmptrader/serf,jmptrader/serf,pmenglund/serf,FollowMyDev/serf,hashicorp/serf,minhkiller/serf,yonglehou/serf,yonglehou/serf,curtiszimmerman/serf,mbrukman/serf,pmenglund/serf,mapuri/serf,gnawhleinad/serf,curtiszimmerman/serf,jmptrader/serf,yonglehou/serf,pmenglund/serf,curtiszimmerman/serf,minhkiller/serf,samaitra/serf,mbrukman/serf,FollowMyDev/serf,patrickviet/serf,minhkiller/serf,gnawhleinad/serf,jmptrader/serf,dalehamel/serf,curtiszimmerman/serf,dalehamel/serf,gnawhleinad/serf,curtiszimmerman/serf,mapuri/serf,hashicorp/serf,mbrukman/serf,mbrukman/serf,yonglehou/serf,gnawhleinad/serf,patrickviet/serf,dalehamel/serf,gnawhleinad/serf,mapuri/serf,dalehamel/serf,minhkiller/serf,minhkiller/serf,gnawhleinad/serf,mbrukman/serf,FollowMyDev/serf,curtiszimmerman/serf,samaitra/serf,jmptrader/serf,dalehamel/serf,FollowMyDev/serf