CASSANDRA-21539: Remove hostname from ClientsTable and GossipInfoTable to avoid reverse-DNS lookups

[nivykani]

The current code uses getHostName(), which has the known effect: This method may trigger a name service reverse lookup if the address was created with a literal IP address. This field in both ClientsTable and GossipInfoTable was constructed with addresses to begin with, not names, so getHostName() was always doing a blocking reverse-DNS lookup on row reads for the first call.

This is a potentially slow blocking call, and we want to avoid doing any DNS lookups here because repeatedly querying this table could potentially overwhelm DNS, especially with frequently-changing client connections.

We could instead use getHostName, but the primary key of both ClientsTable and GossipInfoTable is (address, port), where address is type inet. Thus, this field is redundant.

To the best of my searching, no in-repo consumer depends on this column having a resolved name (and instead use an address-only path), and nodetool clientstats already doesn't display the hostname column.

The Cassandra Jira

[aweisberg]

InetAddressAndPort has a toString() method that is good for splitting the difference of including a hostname along with an IP without doing a reverse lookup that could be copied for other uses

Since we have the column it's a good time to figure out how we want to handle this in the future and I think it's a pick your poison sort of thing. Make it configurable with 1. Reverse DNS lookup 2. Display the hostname if it's there otherwise null column value, and 3. Always set to null.

This would be a good global knob to have for all the system tables and interfaces so we can let people select the behavior they want and it can all route through one wrapper method that consults the configuration and then given an address gives you the right output.