database: expose SetConnMaxLifetime as config setting

For MySQL, setting this to be shorter than the wait_timeout MySQL setting
solves the issue with connection errors after the session has timed out for
the connection to the database via xorm.

conf/sample.ini

@@ -90,6 +90,9 @@
 # Max conn setting default is 0 (mean not set)
 ;max_open_conn =
 
+# Connection Max Lifetime default is 14400 (means 14400 seconds or 4 hours)
+;conn_max_lifetime = 14400
+
 # Set to true to log the sql calls and execution times.
 log_queries =