Add a conf for enable/disable majority-add.

RATIS-1912 added a restriction on setConf for enforcing a safety property, otherwise, the server group may end up in a no-leader state and cannot recover by themselves. The restriction is to disallow majority-add as defined below

• Majority-add: adding a majority of server in a single setConf.  Note that, when a single setConf removes and adds members at the same time, the majority is counted after the removal.

This JIRA is to add a conf for enable/disable majority-add since, in some monitored environment, the safety property is not a problem. The monitor (a human or a program) could intervene to fix the problem, if it happens. Disabling the restriction can improve performance. For example, change from non-HA single server to a HA 3-server group can be done in a single setConf, instead of 2 setConfs with the restriction.