CXX-508: Export logging related symbols

Exported symbols of LogDomain, LogSeverity and MessageEventEphemeral
classes in order for the apps to be able to intercept mongo client's logs

Author: ioancea

src/mongo/logger/log_domain.h

@@ -47,7 +47,7 @@ namespace logger {
      * messages.
      */
     template <typename E>
-    class LogDomain {
+    class MONGO_CLIENT_API LogDomain {
         MONGO_DISALLOW_COPYING(LogDomain);
     public:
         typedef E Event;

src/mongo/logger/log_severity.h

@@ -18,6 +18,7 @@
 #include <iosfwd>
 #include <string>
 
+#include "mongo/client/export_macros.h"
 #include "mongo/base/string_data.h"
 
 namespace mongo {
@@ -29,7 +30,7 @@ namespace logger {
      * Severities are totally ordered, from most severe to least severe as follows:
      * Severe, Error, Warning, Info, Log, Debug(1), Debug(2), ...
      */
-    class LogSeverity {
+    class MONGO_CLIENT_API LogSeverity {
     public:
         //
         // Static factory methods for getting LogSeverity objects of the various severity levels.

src/mongo/logger/message_event.h

@@ -29,7 +29,7 @@ namespace logger {
      *
      * Used and owned by one thread.  This is the message type used by MessageLogDomain.
      */
-    class MessageEventEphemeral {
+    class MONGO_CLIENT_API MessageEventEphemeral {
     public:
         MessageEventEphemeral(
                 Date_t date,