[rpc] validate security-related parameters earlier

This patch removes the verification of a few security-related RPC flags
from the connection negotiation time (i.e. DoServerNegotiation()) since
that's already done in one of the group flag validators.

Apparently, this way it's more optimal:
  * misconfiguration are spotted earlier: if this type of issue
    is detected, the outcome is visible and actionable outright since
    the server simply refuses to start, reporting on the issue
  * there isn't a way to have a Kudu server running with such an issue
  * some amount of CPU resources is spared since a server no longer
    performs the same verification over and over again when establishing
    a new RPC connection, but performs it once during the start-up time

Change-Id: Ie16537c440041c9ee74276da35f2599592ef7e04
Reviewed-on: http://gerrit.cloudera.org:8080/20961
Tested-by: Alexey Serbin <alexey@apache.org>
Reviewed-by: Abhishek Chennaka <achennaka@cloudera.com>
3 files changed