be consistent in visible messages about DOWN versus dead state

Cassandra has two states that seem to be used interchangeably in some messages.

1) "DOWN" state, which simply means that the node has not responded in the last Gossip round.
2) "dead" which means a state listed in DEAD_STATES in Gossiper.java.

static final List<String> DEAD_STATES = Arrays.asList(VersionedValue.REMOVING_TOKEN, VersionedValue.REMOVED_TOKEN,VersionedValue.STATUS_LEFT, VersionedValue.HIBERNATE);

However, it seems to use the terms incorrectly in the following places :

a) the log message :
"
logger.info("InetAddress {} is now dead.", addr);
"

this is in Gossiper::markDead (which ideally would also be renamed, because it does not seem to put anything in one of the DEAD_STATES..)

but in Gossiper::markAlive the paired log message is :
"
logger.info("InetAddress {} is now UP", addr);
"

If being marked alive means you are UP, then being marked dead should mean you are DOWN.

b) NodeToolHelp.yaml has :
"
Disable gossip (effectively marking the node dead)
"

Attached is a patch against trunk which changes both "dead"s to "DOWN".