Breaking up SERVER-49379 into smaller tickets. This is the first.

Like serverStatus metrics for elections, create a class ApplicationApiVersionMetrics. An instance of this class will be stored as a decoration on service context. Use ReplicationMetrics as reference. 

The class should be populated with the field “appNameVersionTimestamps” of type: map<applicationName, <apiVersion, latest-timestamp>>. Define a mutex as a private field in the class. Lastly, add a member function addVersionTimestamp(applicationName, apiVersion) which acquires the mutex and writes a timestamp to an (applicationName, apiVersion) pair in "appNameVersionTimestamps". 

In execCommandDatabase of service_entry_point_common.cpp and in runCommand of strategy.cpp, invoke “addVersionTimestamp”.