MXS-2325 Only enable events that were enabled on the master

The monitor now continuously updates a list of enabled server events. When
promoting a new master in failover/switchover, only events that were enabled
on the previous master are enabled on the new. This avoids enabling events
that may have been disabled on the master yet stayed in the SLAVESIDE_DISABLED-
state on the slave.

In the case of reset-replication command, events on the new master are only
enabled if the monitor had a master when the command was launched. Otherwise
all events remain disabled.

server/modules/monitor/mariadbmon/server_utils.hh

@@ -13,6 +13,7 @@
 #pragma once
 #include "mariadbmon_common.hh"
 
+#include <unordered_set>
 #include <string>
 #include <vector>
 #include <maxbase/stopwatch.hh>
@@ -209,7 +210,8 @@ public:
     bool                      should_be_copied(std::string* ignore_reason_out) const;
 };
 
-typedef std::vector<SlaveStatus> SlaveStatusArray;
+using SlaveStatusArray = std::vector<SlaveStatus>;
+using EventNameSet = std::unordered_set<std::string>;
 
 enum class OperationType
 {
@@ -239,8 +241,15 @@ public:
     const std::string      sql_file;        // Path to file with SQL commands to run during op
     const SlaveStatusArray conns_to_copy;   // Slave connections the target should copy/merge
 
+    const EventNameSet events_to_enable; // Scheduled event names last seen on master.
+
     ServerOperation(MariaDBServer* target, bool was_is_master, bool handle_events,
-                    const std::string& sql_file, const SlaveStatusArray& conns_to_copy);
+                    const std::string& sql_file, const SlaveStatusArray& conns_to_copy,
+                    const EventNameSet& events_to_enable);
+
+    ServerOperation(MariaDBServer* target, bool was_is_master, bool handle_events,
+                    const std::string& sql_file);
+
 };
 
 /**