Remove unnecessary DNS reverse lookups from RPC layer

RPC connection authorization needs to verify client's Kerberos principal name matches what specified for the protocol. For service clients like DN's, their Kerberos principal names can be specified in the form of "datanode/[email protected]". To get the expected
client principal name, the server needs to substitute "_HOST" with the client's fully qualified domain name, which requires a reverse DNS lookup from client IP address. However, for connections from clients whose principal name are either unspecified or specified not using the "_HOST" convention, the substitution is not required and the reverse DNS lookup should be avoided. Currently the reverse DNS lookup is done for all clients, which could slow services like NN down, when local named cache is not available.