better postgres connection pool management

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

weed/command/scaffold.go

@@ -160,6 +160,7 @@ schema = ""
 sslmode = "disable"
 connection_max_idle = 100
 connection_max_open = 100
+connection_max_lifetime_seconds = 0
 
 [postgres2]
 enabled = false
@@ -181,6 +182,7 @@ schema = ""
 sslmode = "disable"
 connection_max_idle = 100
 connection_max_open = 100
+connection_max_lifetime_seconds = 0
 
 [cassandra]
 # CREATE TABLE filemeta (