better mysql connection pool management

adding SetConnMaxLifetime configuration (https://golang.org/pkg/database/sql/#DB.SetConnMaxLifetime)
to enable refresh of connections.

weed/command/scaffold.go

@@ -118,6 +118,7 @@ password = ""
 database = ""              # create or use an existing database
 connection_max_idle = 2
 connection_max_open = 100
+connection_max_lifetime_seconds = 0
 interpolateParams = false
 
 [postgres] # or cockroachdb

weed/filer/mysql/mysql_store.go

@@ -35,12 +35,13 @@ func (store *MysqlStore) Initialize(configuration util.Configuration, prefix str
 		configuration.GetString(prefix+"database"),
 		configuration.GetInt(prefix+"connection_max_idle"),
 		configuration.GetInt(prefix+"connection_max_open"),
+		configuration.GetInt(prefix+"connection_max_lifetime_seconds"),
 		configuration.GetBool(prefix+"interpolateParams"),
 	)
 }
 
-func (store *MysqlStore) initialize(user, password, hostname string, port int, database string, maxIdle, maxOpen int,
-	interpolateParams bool) (err error) {
+func (store *MysqlStore) initialize(user, password, hostname string, port int, database string, maxIdle, maxOpen,
+    maxLifetimeSeconds int,	interpolateParams bool) (err error) {
 	//
 	store.SqlInsert = "INSERT INTO filemeta (dirhash,name,directory,meta) VALUES(?,?,?,?)"
 	store.SqlUpdate = "UPDATE filemeta SET meta=? WHERE dirhash=? AND name=? AND directory=?"
@@ -65,6 +66,7 @@ func (store *MysqlStore) initialize(user, password, hostname string, port int, d
 
 	store.DB.SetMaxIdleConns(maxIdle)
 	store.DB.SetMaxOpenConns(maxOpen)
+	store.DB.SetConnMaxLifetime(maxLifetimeSeconds*time.Second)
 
 	if err = store.DB.Ping(); err != nil {
 		return fmt.Errorf("connect to %s error:%v", sqlUrl, err)