Support TLS-only ZK server

[abhilash1in]

This PR addresses ZOOKEEPER-4276, which enables a cluster to run as a TLS-only cluster on the secureClientPort.

Previously, this was not possible due to reasons mentioned in ZOOKEEPER-4276.

In this change, we introduce a new section to sever.X entries of dynamic config to indicate secureClientPort. We also make clientPort section of the dynamic config as optional (previously, this was mandatory).

For example:

server.5 = 125.23.63.23:1234:1235;;1237

does not have a clientPort and has 1237 as the secureClientPort.

Dev mailing list thread - https://lists.apache.org/thread/1lmqj2wonldxmk3wxbc5lvj3vsz05l7c