Code cleanup in MySQL_Thread::refresh_variables()

Use macros to avoid repeating variable name and strings with the variable name.

Because this commit avoids duplications of name and literals, this commit also renames:
mysql_thread___monitor_groupreplication_max_transaction_behind_for_read_only
to:
mysql_thread___monitor_groupreplication_max_transactions_behind_for_read_only

include/proxysql_structs.h

@@ -916,7 +916,7 @@ __thread int mysql_thread___monitor_groupreplication_healthcheck_interval;
 __thread int mysql_thread___monitor_groupreplication_healthcheck_timeout;
 __thread int mysql_thread___monitor_groupreplication_healthcheck_max_timeout_count;
 __thread int mysql_thread___monitor_groupreplication_max_transactions_behind_count;
-__thread int mysql_thread___monitor_groupreplication_max_transaction_behind_for_read_only;
+__thread int mysql_thread___monitor_groupreplication_max_transactions_behind_for_read_only;
 __thread int mysql_thread___monitor_galera_healthcheck_interval;
 __thread int mysql_thread___monitor_galera_healthcheck_timeout;
 __thread int mysql_thread___monitor_galera_healthcheck_max_timeout_count;
@@ -1087,7 +1087,7 @@ extern __thread int mysql_thread___monitor_replication_lag_count;
 extern __thread int mysql_thread___monitor_groupreplication_healthcheck_interval;
 extern __thread int mysql_thread___monitor_groupreplication_healthcheck_timeout;
 extern __thread int mysql_thread___monitor_groupreplication_healthcheck_max_timeout_count;
-extern __thread int mysql_thread___monitor_groupreplication_max_transaction_behind_for_read_only;
+extern __thread int mysql_thread___monitor_groupreplication_max_transactions_behind_for_read_only;
 extern __thread int mysql_thread___monitor_groupreplication_max_transactions_behind_count;
 extern __thread int mysql_thread___monitor_galera_healthcheck_interval;
 extern __thread int mysql_thread___monitor_galera_healthcheck_timeout;

lib/MySQL_HostGroups_Manager.cpp

@@ -2866,8 +2866,8 @@ void MySQL_HostGroups_Manager::group_replication_lag_action(
 		MyHGC* myhgc = nullptr;
 
 		if (
-			mysql_thread___monitor_groupreplication_max_transaction_behind_for_read_only == 0 ||
-			mysql_thread___monitor_groupreplication_max_transaction_behind_for_read_only == 2 ||
+			mysql_thread___monitor_groupreplication_max_transactions_behind_for_read_only == 0 ||
+			mysql_thread___monitor_groupreplication_max_transactions_behind_for_read_only == 2 ||
 			enable
 		) {
 			if (read_only == false) {
@@ -2877,8 +2877,8 @@ void MySQL_HostGroups_Manager::group_replication_lag_action(
 		}
 
 		if (
-			mysql_thread___monitor_groupreplication_max_transaction_behind_for_read_only == 1 ||
-			mysql_thread___monitor_groupreplication_max_transaction_behind_for_read_only == 2 ||
+			mysql_thread___monitor_groupreplication_max_transactions_behind_for_read_only == 1 ||
+			mysql_thread___monitor_groupreplication_max_transactions_behind_for_read_only == 2 ||
 			enable
 		) {
 			myhgc = MyHGM->MyHGC_find(reader_hostgroup);