Add support of performance settings like 'SET GLOBAL max_allowed_packet=1073741824'.

conf/borg2-backup.conf.sample

@@ -130,6 +130,26 @@
 #MYSQL_CREDENTIALS="-u root -S /run/mysqld/mysqld.sock"
 
 
+# MYSQL_PERFORMANCE_SETTINGS
+#
+# To increase performance, some global variables are changed before writing the database dump.
+#
+# Settings are:
+#     - SET GLOBAL autocommit='OFF'
+#     - SET GLOBAL foreign_key_checks='OFF'
+#     - SET GLOBAL unique_checks='OFF'
+#     - SET GLOBAL innodb_flush_log_at_trx_commit=2
+#     SET GLOBAL max_allowed_packet=1073741824
+#
+# After the databases have been backed up, the values are reset.
+#
+# Defaults to:
+#
+#     MYSQL_PERFORMANCE_SETTINGS=false
+#
+#MYSQL_PERFORMANCE_SETTINGS=false
+
+
 # PSQL_SYSTEM_USER
 #
 # User who owns PostgreSQL services