MXS-2592 Add configuration for session specific in-memory log

When enabled each session will write log messages in the in-memory log.
If session ends in error this log is written to the actual log in disk.

maxutils/maxbase/include/maxbase/maxbase.hh

@@ -78,7 +78,8 @@ public:
             const char* zLogdir,
             const char* zFilename,
             mxb_log_target_t target,
-            mxb_log_context_provider_t context_provider);
+            mxb_log_context_provider_t context_provider,
+            mxb_in_memory_log_t in_memory_log);
 
     /**
      * @brief Initializes MaxBase and the MaxBase log.
@@ -88,7 +89,7 @@ public:
      * @throws std::runtime_error if the initialization failed.
      */
     MaxBase(mxb_log_target_t target)
-        : MaxBase(nullptr, ".", nullptr, target, nullptr)
+        : MaxBase(nullptr, ".", nullptr, target, nullptr, nullptr)
     {
     }