Delete old checkpoints in stateDB automatically

Introduce a constructor parameter for the StateDB called `keep`, which tells
how many checkpoints to keep.  When doing a new checkpoint, if the number of
existing checkpoints exeeds `keep`, the oldest ones will be deleted.

config/config.go

@@ -102,7 +102,8 @@ type Node struct {
 		Type     string   `valudate:"required"`
 	} `validate:"required"`
 	StateDB struct {
-		Path string
+		Path string `validate:"required"`
+		Keep int    `validate:"required"`
 	} `validate:"required"`
 	PostgreSQL struct {
 		Port     int    `validate:"required"`