ExportedServerParameter does not protect the underlying templated variable from concurrent access.

The following construct is hence not safe if maxConsecutiveFailedChecks is ever read without holding a lock preventing a setParameter call:

ExportedServerParameter<int> MaxConsecutiveFailedChecksSetting(                                             
     ServerParameterSet::getGlobal(),                                                                        
     "replMonitorMaxFailedChecks",
     &ReplicaSetMonitor::maxConsecutiveFailedChecks,                                                         
     false,  // allowedToChangeAtStartup                                                                     
     true);  // allowedToChangeAtRuntime

We could either:
1) Prevent ExportedServerParameter to be configured with allowedToChangeAtRuntime = true, and make implementers having to inherit from ServerParameter directly, or

2) Use a mutex in ExportedServerParameter or use atomics for the template specializations where that make sense.

Regardless we should audit the existing uses of ExportedServerParameter.